Do Falcon 9 grid fins lock up when extended during reentry? Or do they rely on aerodynamic forces to keep them extended once they're deployed? Could the fins flail down during the reentry burn when thrust acceleration is high (>2 g's)?
If you look at the webcasts you'll see that they are extended while in free fall (before re-entry) and you can kind of see them bounce and lock into place with a jolt. I believe they are hydraulically actuated and do not rely on aerodynamic force to stay extended. I do not know about your second question however.
See the OTV5 / X-37B launch from 2017 just after the boost-back burn (@5:47 in linked video, ~T+3:35 mission clock): OTV5 / X-37B Booster Cam
As you can see they are deployed above 100km altitude so at least initially there are no aerodynamics at play.
You raise an interesting question as to how much help the supporting structure gets from aerodynamic forces while doing the landing burn, anyone else have ideas on that?