Trouble is if we assume that the apparent cooling (from 3000 K to 2.7 K) is due to relativistic doppler effect alone, then we need to attribute a recession velocity close to the speed of light to the surface last scattering that is presenting itself to us.So if that surface is both now 13.8 billion lt-yrs away and was travelling near the speed of light when the radiation was emitted, then we have a universe that is about twice as old as the generally accepted age, or alternatively, that the radiation from the CMB is arriving at us from half as far (6.9 billion lt-yrs instead of 13.8).

There's a ton of science that goes into analyzing the CMB and determining the age of the observable universe, I suggest starting with the insights written by @bapowell.

This is not correct. First, the geometry of spacetime isn't being "altered" by expansion; "expansion" just means the 4-dimensional geometry of spacetime, which doesn't "change", it just is, has a certain shape.

Second, gravity doesn't "rearrange" the geometry of spacetime; as above, that geometry just is, it doesn't change. "Gravity" is just one way of describing certain effects of the geometry of spacetime.

Relativistic doppler is not a good way of thinking about the effect of curved spacetime geometry on light rays. Relativistic doppler is really only a workable model for the case of flat spacetime and a light source and receiver that are in relative motion.

It didn't.

Also, please review the PF rules on personal speculation, which is what the rest of your post after the above quote is.

None of this is correct. You need to take the time to learn the correct model from a textbook; a detailed explanation of the model is well beyond the scope of a "B" level thread.