Ok, not getting any responses, so I'll provide the answers to this question, spoiler space follows, don't scroll down if you don't want to read this.
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For the second question the weight increases the same as the weight of the air added. The result is 32lbs, 26lbs from the tank, 6lbs more from the air inside.
The real question is how to gases exert their weight within a closed system. The answer is pressure differential. Pressure decreases with altitude within a closed system acted upon by gravity (or equivalent acceleration). Using the scuba tank as an example, the pressure at the top is less than that the pressure at bottom, with a net downforce exactly equal to the weight of the gas inside the tank. It doesn't matter what the shape of the tank is, or how it's oriented, the pressure differential within the tank always results in a net downforce equal to the weight of the gas inside the tank.
The same thing applies to the original closed system. Let's remove the object and add 1 lb of air so we have a 50 lb box with 50lbs of air in it. Again, the pressure differential will create a net downforce within the box of 50lbs, exactly the weight of the gas inside the box. Again, the shape, size, or orientation of the box doesn't matter, (as long as the size isn't so large that changes in the pull of gravity or cetripetal reaction to rotation of the Earth isn't an issue), there will always be a net downforce equal to the weight of the gas within the box. One factor to note is that the smaller the box, the higher the pressure and denstiy of the gas inside, the rate of pressure change versus altitude is greater than that of a larger lower pressure, lower density gas filled box.
Getting back to the flying / gliding object case, we have 50lbs of box, 49lbs of air, and 1 lb of object. Well it's a closed system, gravity doesn't stop pulling down on the object just because it's flying. As long as there are no net vertical acceleration, the system will always weigh 100lbs. So while the object is flying/gliding, it has to create a pressure differntial that creates a net downforce of 1 lb within the box. This is one way to argue that wings produce lift by accelerating air downwards.
In the case of the helium balloon. Let's start with the balloon defalated and with the helium inside a tank on the model balloons gondola. As the balloon is inflated with helium, the pressure within the box increases. As the pressure within the box increases, so does the density, which in turn, increase the pressure diffferential. Once the balloon is hovering, the density of the surrounding air has been increased so that the higher density air's pressure differential results in a 1lb gain in net downforce, exactly equal to the weight of the helium balloon model. If the helium balloon is continued to be inflated, it rises upwards into lower density (lower pressure) air and hovers again. The over all increase in pressure remains constant as long as the balloon is hovering. The balloon can continue to be inflated to increase the pressure that creates a differential of more than 1 lb, but in this case it's risen to the top of the box, and pushes upwards at the top, exactly canceling out any further gain in net downforce due to increase in air density within the box.
Lastly is the magnetic case, which isn't as intersting, since it simply pulls downward on the top of the box.
In all cases, as long as there is no vertical acceleration of the center of mass of the closed system, the weight of the system is constant.
Once the birds flying in a plane end their vertical acceleration, the weight of the system remains the same as before when the birds were resting.
