When someone tries to cross me up after knockdown oki when I am using Kula, I use her light Crow Bites(623A) (6 frame startup) in the opposite direction. Heavy Crow Bites (623C) is faster but has No Invincibility afaik. You can use the EX Crow Bites (623AC) instead since that one is 2 frames faster, is also invincible, and does 123 more more damage but I would rather spend her power meter in the corner where you get easy safe jump setups out of it (rather than just damage) unless you recognize that using EX Crow Bites will kill or that you need the increased speed. Since it's hard/impossible for a crossover attack to be safe from wakeups, Light Crow Bites with its 6 frame startup has been fast enough for me from my experience. However, If your opponent is really good at landing their crossups deep enough (like safe jumps) to block Light Crow Bites, I recommend adjusting to using EX Crow Bites. Watch out for jumps done so early they land before you get up though; those are used to bait wakeup DPs, but on the plus side, those kinds of jumps won't be able to hit you on the way down so you don't have to worry about any blockstun pressure from the jump itself.
I think it's better to counter crossups with invincible/antiair moves rather than block them because they'll get block pressure on you. It seems even more important to do this against characters like Iori, EX Iori, and Kyo since their air command normals have large hitboxes and cause outrageously long hitstun/blockstun; better players can hit confirm off of these crossover attacks by themselves (one hit confirms). Not challenging the crossup is giving them freedom to go for the crossup again and again, or mixup (maybe a followup hop C or empty crossover jump into low or command grab, frametrap, etc. I do hate playing against EX Iori though.
Rolling towards the direction EX Iori jumped from when he uses a badly spaced taco is usable for every character in the game; If Iori spaces and times the jump lazily, he may land too far from you to punish you with run-up close C, far C, DM, or EXDM, but rather than rolling, Kula's invincible Crow Bites (A or EX version) is a much better option imo.
Doing the anti-crossup Crow Bites the other way is easiest on wakeup when you can gauge opponent's height properly and know if their jump can be hit by Crow Bites or their jump is timed so early it's an attempt to bait you.
It's a lot harder to do when someone gets enough pressure on you to jump at you while you're on your feet though. There's a tricky vulnerability window to deal with between blocking them high in front and then doing the Crow Bites the other way as they cross over because it's hard to recognize when they've crossed over... Keep in mind that the same direction you're holding to block is going to be used for the starting motion of anti-crossup Crow Bites. You can try it out but I suggest stopping if it isn't working out for you and switching to rolling. Rolling is actually much safer against jumps when you're already on your feet since you can roll on reaction earlier during their jump (as opposed to when they have wakeup oki on you and they're already on the way down); they will take longer to land so they will probably not be able to punish you as easily (if at all).
In short:
1) Consider wakeup Crow Bites against unsafe crossups on wakeup. (Any character with an invincible fast antiair can do this)
2) Use rolls on reaction against close range jumps if you are free to move (on your feet, not in hitstun/blockstun, and no slow projectiles are in your way)