for manaka's specials, book toss is the projectile, it has 6 different arcs counting jumping ones, you can jump over the light and medium ground ones and run under the heavy ground one. The splash on the books is random if a Manaka jumps in at you and throws books behind you, sometimes the splash will hit you out of whatever punish you thought you could get and then manaka will get a combo.
The counter move has pretty significant start-up and you can hit her as long as she isn't behind the bookshelf (also she can be command grabbed out of the bookshelf, possibly normal grabbed too, not sure). Also if manaka has you cornered she can use this move to teleport a full screen away (I don't think I've ever seen anyone do that on purpose though).
the move where she tips over a bookshelf is primarily a combo ender. It can sometimes be used as an anti-air I think, but not very commonly.
the stumble thing is her dash, it's basically her only way of moving forward since her forward walk speed is effectively non-existent. Out of it she can do a low, an overhead, or a launcher (which I'm pretty sure is air unblockable). The low is fast and can be used to start pressure, so that's her most likely option. The overhead is punishable on block unless covered by an assist and only leads to a combo on counter hit or if they spend meter on an assist to pick you up. The launcher is super punishable and mainly used either for combos or to catch people jumping away from the stumble.
A few moves can go right into the stumble (BC and low emotion book shelf super) on hit or block. Also Manaka can't block while stumbling.
To get out of the 2b > 2b pressure you can guard cancel after the first 2b and if the Manaka is just trying to do 2b all day they'll get hit. If you do it after the followup though she has enough time to block. Also I know Chizuru can backdash out, probably other characters can too.
