check and mostly fold based on sizing. You arent really getting exploited here, unless he has AsKJT or similar, he isnt going to be bluffing here. And yeah, he should be never shoving a worse made hand here

1. You're thinking along the right lines (2nd best hands don't need protection, they need free cards).

2. Correct, as a default against a sane opponent. And he can also bluff a certain % (pot-odds based) and you'll still have to fold. But all is not lost after a check. Sometimes he checks back or bets small.

3. Always check-folding (with your range) is certainly exploitable. However, if you often find yourself in this situation, you might be check-raising too wide on drawy boards. So you control this to some degree on the previous street by having a check-raising range that can continue on lots of turns. Top set + BD nutflush draw certainly can, and when the worst turn cards come, you just try to play well.

However, you can consider check-calling your driest sets on the flop as well, if you expect Villain to barrel lots of turns (autobetting reduces his positional advantage, and you can exploit that with slowplays). If the turn comes bad, you can check-call with showdown value (he'll not always bet turn + river) + some implied odds, and then play the river. If the turn comes blank, you can check-raise.

In Hold'em, a set is a set. In PLO there is a huge difference between a dry set and the same set + strong draw, and it can be a good idea to sometimes play the dry ones slower (especially OOP and multiway on drawy boards with high SPR) and wait for the turn to define your hand better. A dry nut/top set can of course always get it in on the flop, but if you think it's a good idea to mix it up heads-up and OOP against an aggressive opponent, feel free to do so (whereas multiway, you'd always try to jam it in).

Especially deep and on very drawy boards this is a concern, even with a set. You'd like to avoid sticky turn spots like this against a good and aggressive player (who will not mindlessly stack off on the flop, but call your c/r a lot and play position over the next two streets). Deep and OOP, PLO becomes very much a game of redraws. Your c/r range therefore needs to be tighter.

As played, you can throw in the occasional strong turn check-call, if you're concerned with balance after checking. You'll have flushes enough in your range, and with them you're not concerned about giving free cards to worse hands (if he had a set, he'd usually jam the flop). A non-nut flush will be very WA/WB here and you can check to induce bluffs without much downside, and this will protect your weak checks as well. If he checks back, you can still get stacks in on the river.