I'm not an expert, but I'll try to anwser your questions with my best knowledge of general relativity. I hope you like reading...
CarstenDierks said:
For me it is difficult to understand why gravitational waves (gravitons) should propagate through spacetime with c and, thus, cause space curvature (if I have understood that correctly).
Yup your're gettin it. I guess you could say that gravitational waves cause space curvature, though you could say vice versa, and as well that gravitational waves
are space curvature. They propagate at c because they have rest mass zero as does a photon.
CarstenDierks said:
Difficult because of the following:
(1) If gravitons (gravitational waves) are not at rest in a gravitational field, this would imply to me that mass needs to constantly emit gravitational waves (gravitons) to "replace" those which are "gone".
Not exactly so. First let's consider gravitational waves as a force through spacetime. Let's almost forget about graviton particles for they are very hypothetical and are just what make up the gravitational wave. So we don't say gravitation is emited, it's exerted. We know that when space is curved by mass, it will "deforme" all of space, most anologically spacetime fabric.
Like the rock in the pond, when we throw the rock in, ripples will not be continuesly exerted forever, but the ripples that were exerted along the body of water will continue traveling as so until they barely have a wavelength (or are slowed or stopped by more ripples of another force). Once the rock has hit bottom, it's deformation towards the body of water is finished. Now the water will adapt to have this new rock in its pond. Translation:

the second a star is formed in space it deformes (curves) spacetime fabric, and by doing so it effects spacetime by sending gravitational waves along space forever. Once the star is formed and balanced in space, it's job is done, and space has adapted to the deformation of this new star. So the star is the rock, the gravitational waves are the ripples and spacetime fabric is the water. So gravitational waves are not constantly emited, it happens once and travels as a wave at the speed of light. Those which are "gone" have already done their job of curving space. They will keeping doing their job forever, but always at further and futher parts of the cosmos. It is now space curvature (created by the wave) to do it's job of creating and exerting gravity. (Think of gravitational waves through the water anology, but don't think of it as what gravity is, because you can't sink to the bottom of spacetime. The bowling ball anology is good for gravity.)
CarstenDierks said:
(2) If mass curves spacetime: Is the curvature once "engraved" in spacetime and "rests" there until a new gravitational wave "updates the information"?
Yes. Engraved is a great word. But the altering of a star's gravitational field is usually very minimal because stars are far away from each other, so there is minimal effect. But it happens and I will give an example soon.
CarstenDierks said:
Or:
(3) Is the curvature of spacetime just "newly" evoked by every ripple of a gravitational wave passing by?
Ill say no, but I don't exactly understand the question. Once a gravitational wave has passed a certain spot, that spot is officially considered effected by the mass that sent the wave off. The only way that the certain spot can be effected again by the same mass is if the mass or density were to change (of the mass) or if the mass would disapear, "taking it's wave back". What I mean by this is that if the object were to disapear it would send of a wave that would un-deforme space.
And remember that all this doesn't happen at once, the waves have a finite speed "c", and each spot is affected at different times.
CarstenDierks said:
(4) Do gravitational waves interfere with each other? Probably yes because gravitational forces and the curvature of space of two objects do add up.
Right. Right. Here's what I said I would get to. In our solar system, planet's orbits are elliptical, they are eccentric. There are debatable reasons for why, some say it's because planets don't orbit the sun, they orbit the common center of mass between the sun and that planet, some say that other gravitational force of other planets effect orbits, mostly Jupiter. And Jupiter is a great example for more understanding. Because it's so big it has a much stronger force of gravity i.e it curves spacetime more. (this is why it has 63 moons and probably more. More objects are likely to fall in orbit around Jupiter because of its strong gravitational force.) So now picture the sun sending a gravitational wave and Jupiter doing the same. The sun and Jupiter (in astronomical terms) are very close to each other. So the two waves sent of by each mass will clash together. Once the clash is finished with, spacetime has adapted to the deformation. It's engraved.

Now in the middle of this weird deformation of space (between the sun and Jupiter), we've got planets. The sun obviously has a stronger gravitational force, so the planets orbit the sun, but they are still effected by Jupiter's gravity. Making their orbits eccentric. (This is not completely proven, but today it is said that Jupiter probably has an effect in our solar system.) (And aswell, planet's past Jupiter still feel the effect of both the sun's and Jupiter's gravity) So yes. Gravitational waves interfere with each other.
*The reason planets orbit is around bigger mass is because they start off with velocity.Iif we were to slightly place a planet in space near a star in would be forced towards the star at an accelerated speed (dependent on the mass). "In would fall down the slope". In a black hole, "it would fall into the hole and be stuck".
CarstenDierks said:
Arising questions:
(5) Is it allowed to conclude out of (4) and (1) that the path of gravitons is not straight but also influenced by the curvature of spacetime (of other objects)? Meaning: Gravitons (gravitational waves) have to travel along our (curved) cosmos as it exists?
Ya, because all large object's are symmetric. They are all completely 3-Dimensional, and will exert gravitational waves (gravitons) in every direction of the cosmos.
CarstenDierks said:
(6) Out of (4): What about gravitational forces of 2 objects of identical mass on a 3rd object right in-between the two? Is the gravitational force for the 3rd object zero? But is spacetime not curved at that point due to the sum of the curvature of the first two objects?
Let's make sure that the objects have the same density also.
This is a complicated question and is being studied today by trying to understand what's going on in binary star systems and star clusters. But I'll say this, because the question is answerable. As long as the objects are sperated by inches, centimeters, millimeters, they will both exerted the same gravitational wave (because they are the same mass and density). But when these waves "clash" together they will have a different overall curvature of space. Possible equal to the sum of each of their gravitational forces, but not exactly sure. But if we put an object in the middle I'm stumped. One would have to study the motion of this 3rd object a lot to understand what kind of pattern (orbit) it would follow, and why is that so. It's being studied as we speak. There are a lot of experts on this site that might have a basic answer, but it is a complicated matter.
CarstenDierks said:
(7) Out of (6): So are gravitons (gravitational waves) and the curvature of spacetime really equal?
Let's put it this way. Gravitational waves (gravitons) curve spacetime at equal magnitudes.
CarstenDierks said:
(8) Out of (2): Is this true for black holes? Do they curve spacetime and the curvature "rests" there because the gravitons (gravitational waves) cannot escape from inside of the black hole? Is the gravitational field of black holes never "updated" by gravitons (gravitational waves)?
None of this happens. The curve rests with every object in space wherther it's a black hole or pluto. The same as any object in space, once it's there, it's there with the speed of light, then it's engraved in space. Your question does makes sense though. But the let's say a black hole gained mass. The second it did so it would curve space more, in all directions. The reason the gravitational waves can "escape" the "hole" (the strong force of gravity) is because they travel along space curvature itself. Gravitons aren't exerted by the mass, they are exerted by the
change in mass. Gravitons don't come out of the singularity of a black hole. They come from around.
One slight change is space curvature will effect the next and so on. So picture a gravitational wave in a black hole, it will travel in all directions around the singularity. This wave will curve this spot, that spot will curve the spot right after it and so on. It's one big wave. Not a particle. (String theory is the real use of the graviton.) So as one spot in space is affected (more curved), at the speed of light the next will be affected (curved). Leading to a spacetime wave climbing up the hole, climbing up the space fabric. The spacetime (gravitational) wave created the curve in the first place. You can't say that once the hole has changed (is deformed) the hole itself won't let it. It's already been changed. (The change is due to the change in mass)
CarstenDierks said:
(9) Out of (1), (3), (4), (7) and (8): How can black holes capture gravitons (gravitational waves) inside the Schwarzschild radius and, at the same time, emit gravitons (gravitational waves) to curve space and exert gravitational force?
Think I covered this. I also think you mean the event horizon not the Schwarzschild radius, which is where nothing can escape. Gravitational waves (packets of gravitons) don't have to escape the event horizon, "they go around it". In other words they travel along the curve of a black hole as a wave. They are what put the event horizon there in the first place.
CarstenDierks said:
(10) Out of (9): How does quantum and/or string theory explain the speed and escape speed of gravitons (gravitational waves)?
The speed of a graviton is that of light: "c". Quantum and string theory, uses the graviton's properties and finds that they match those to supersymmetric strings. Example: they both have spin-2. The graviton is a way to incorperated gravity into particle physics and string theory.
Carsten I think your questions are very valid and I understand their meaning. I hope I was able to cover them, and it is a difficult task to both ask, answer and understand what is going on in spacetime curvature of a black hole. Hope I helped
