So, you're saying crotch-leg 2 has a separate material entirely than crotch-leg 1? I'm not familiar with UDIMs so can't speak to that part of your story, but for a traditional workflow, the 0-1 space (the lighter gray square where most of your UV elements are) is the only space that gets unique texture information-- it…
I feel your pain. I had to go through RTS unit models that were horribly unwrapped for a student project and when it came to aligning things I wondered if it this was possible too. In the end, I just used these to speed the process up (since it didn't seem I could use the repeat last command hotkey with the align buttons…