If he is a competent player, he is only raising with value hands that beat you (why should he semibluff into a dead pot).
Then you do not have the right odds to call here and a fold would be the best play.

If you think he is a crazy maniac who does this with rundowns/overpair+flush draw..., you can decide to call or go all in.

this is too loose an iso pre. I don't think you have a choice but to raise pot on the flop with this hand.

Are you suggesting to get 400bb in with bottom 2pair Vs a repot vs a shove from a fish+our call. That's brining money on the highest level. What do you expect him to show that we are doing really good against while we are crushed by sets, higher 2p or wrap (+pair) which we will get shown almost always? We're not even in great shape against 78 + pair. Fold! It's way closer to a fold against utg all in than to a call or even raise against reshove.

I'm nooby in PLO, but here is my thoughts:
- 99xx is 6961 combination, so not so rare (we have 28% EQ against it)
- not so rare as I risking my 450bb stack
- If I didnt see a move like that from this guy, probably I fold and see what happening

This decision is up to your initial plan. If you had been thinking to let the BB player in, this would be reshove. Otherwise, you should raise to the donk. You get bottom two pair and nuts flush draw, it is good enough I guess. Of course, the BB might be 99XX, but it is possible that he gets kind of 6789 or 78TK with flush draw. So, I suppose reshove gonna be the best choice because of out of position, also, you said the BB is not a tough reg.