Is there any unified list of officially undefined things?

That's the only one that comes to mind. It's documented (too specifically) in perlsyn under statement modifiers (my ... if ...;).

I know, for example, by luck, that $i++ + ++$i can officially do whatever it wants

That one's simply undocumented.

I wonder if it might be appropriate for the precedence table to mention this unexpected (by me) interaction?

I don't see why not, although I don't think it'll ever be an issue in legit code (but my brain is too slow to make sure of that right now). Maybe as a warning in the doc for the cond op.