Also if I am paranoid about mirror data being exactly the same on the
two halves (yes, I understand softraid should guarantee it, but
still...), how can I verify it? Or this functionality currently
nonexistent? Or am I asking a stupid question because softraid is
guaranteed to notice these things and yell at me about them? There's
nothing in manual page about this. Can I do it with other utilities?

tl;dr: How to verify softraid mirror integrity?