Tested some stuff with her tonight and she gets stupider (in a good way) with every minute I spend in the lab.
Ukihane (j.d+B) inflicts RIDICULOUS blockstun; I kept trying to get blocked by the dummy in a really unsafe way and with 1-guard jump + standing block in a Mai mirror I was unable to observe any situation where Mai was worse than neutral on block. NEUTRAL! If this move does not flat-out miss the opponent it's 100% safe AFAIK. Hop, full jump, late, early, doesn't even matter. Go nuts with this move if you think it won't miss or get DP'ed/supered. NerdJosh's instincts were right on target.
Mai recovers fast enough from uncancelled corner s.CD to link c.C afterwards. So you don't need to rely on linking from an A Kacho Sen, which is good, because if you're too close when you CD, the fan will miss. And of course raw CD is still plus as hell on block, so there's no consequence except guard cancels and invincible moves for going nuts with it.
Tigerknee Musasabi no Mai is also actually safe to plus on block if you're blocked (stand blocked) any later than the top of the opponent's head (riskier against tall characters, probably). TK right in their face is slightly plus on block, but of course that cries out "please hit me out of the startup". So an occasional shenanigan, no chance of Mai turning into 13 Yuri on the divekick front.
EX charge walldive has projectile invul on startup. Dunno yet if it lasts all the way to the wall since most of the fireball moves in this game are low to the ground. Where's King air Venom Strike when you need it? (RIP)
hcf+K 2nd hit is actually plus on block on crouch guard. Like, significantly so. But that doesn't matter cause all you need to do to avoid that situation is stand up while blocking the 1st hit and then she's -9,999 like you'd expect.