1. Not that I know of. How much RAM do you have? You might be running out of memory...

2. Yes you can have different sized maps. To have your bump lines appear same width and depth you have to do some experimenting.

3. Larger textures do have an effect in the render time. They take longer to load and they take more RAM.

4. You should be able to do this. It will take longer to convert to 2k maps though. Again this sounds like you're running out of mem.

Doing UVs properly on a complex object will take a lot of time. Unfortunately that's just how it is. And if you just bodge them quickly it will show in the end.

My chassis textures are 2k maps. Here's what my UVs look like (scaled down to 512x512):