Regarding PO cancel 1A, it's possible to both roll and block low which will end up making the PO grab and PO B whiff, as well as blocking the 1A. The viable option for Nat here in my opinion is 1B/1K after exiting PO, or alternatively try this:
In that situation, PO B/PO B+K is a mid/low mix-up. If it's not done immediately then it can be stepped. As for the one or two characters who might be able to get out of PO B+K, I wouldn't worry about that. If someone's going for JG on PO B, they will take a PO B+K - if they try to block low for PO B+K, they will take a PO B.
I really wouldn't recommend PO grab because it's a simple B break that everyone can mash and get out of. PO dash cancel then an A grab works for me a lot until it's being figured out, then it's back to a standard A/B grab mix-up. It's easy enough to tell whether Nat's still in PO or if she's exited it, so the PO grab should always be teched. That's not to say that the PO grab will never work, because it can and does, but it is no more than an easily escapable gimmick.
I actually use PO B all the time, I'm known for doing PO B chains, sometimes 3 or 4 in a row if the opponent keeps respecting. It is an under-appreciated move. Keep an eye out for my PO Bs whenever I show up on streams or on my own stream whenever I decide to turn it on. I have many fights where I'm doing PO B every time I'm at advantage if the person I'm fighting isn't dealing with it correctly.
EDIT: Remember that when you do a PO dash cancel during oki, you are basically sacrificing all of the advantage Nat had if the opponent gets up straight away. You could see that as a mix-up in itself; if they don't get up straight away and you exit PO, you are up close after a PO dash with advantage for 1B/1K ground poking. If they do immediately get up, you are up close after a PO dash then PO exit, but at neutral or worse (PO exit recovery frames).