The best way to amswer these questions would be to create a clothed UMA from scratch and see how the system works. It's not an easy system to get your head around but when you do it does make sense... mostly.
Theoretically you can use any shader for an UMA, I recently upgraded the basic UMA system to the HDRP shaders without much difficulty. Using an external shader created with shadergraph is certainly possible from a technical perspective but whether you can do that without the use of an asset bundle I really couldn't tell you off the top of my head.
The whole T-Pose thing... An A-Pose is better from an animation perspective, it allows for better deformatiom and easier weight-painting. The A-Posed model is forced into a T-Pose during import into unity with a file created for the UMA to use. Updating the bone structure or vertex groups without regenerating this file is likely to be what's causing issues.
UMA _only_ works with asset bundles if you're trying to side-load data. The newest version of UMA can use addressables but the last time I played with addressables the whole system was a buggy mess. That was quite a while ago so things can only have gotten better since then.