> Does the committee have any plans to document the rationale for each kind of Undefined Behavior?

In the C99 timeframe, we had a rationale document that was separately maintained. My understanding (this predates my joining the committee) is that this was prohibitively labor-intensive and so we stopped doing it for C11. I don't know of any plans to start doing this again, even in a limited sense for justifying UB. That said, we do spend time considering whether an aspect of a proposal requires UB or not, so the rationale exists in the proposals and committee minutes.

> Does the committee have any plans to make NULL pointer arguments to memcpy non-UB when the size argument is 0?

I have not seen such a proposal, and suspect that implementations may be concerned about losing their optimization opportunities from such a change. (Personally, I'd be okay losing those optimization opportunities as this does not seem like a situation where UB is necessary.)