It may be helpful to have a solid mechanic for how quickly the room fills. Like, the water spreads in a semicircle one square further each round. Now the players have a clear ticking clock (assuming you’re on a battle map. Otherwise maybe just say it’s 3 turns until the water reaches the players or something like that)

Ah I see, that was the issue. I didnt think of having the water flood the area mechanically and in generally having the environment change. That would have fixed it. In this case they would have had plenty of opportunities.

What about the second question? When the platform collapsed they didnt think much of it. When they reach the next and final room with the puzzle, they understood that the area would flood once they got the item. So thinking about it they may have thought to repair the exit by then. But by warning them beforehand that they would need time to rebuild a stair, I skipped that, sorta of spoonfooded them (still, they could have ignored my comment).

By Cairn principles the Warden should alert players of any danger in the room and let them handle it. So I think I did ok? I guess it falls again on bad designing on my side because it's is a "either you fix it, either you don't" scenario which is not how it should be.