I'm prepping to run The Ghost of Mistmoor. It was originally published in Dungeon magazine (#35, May/June 1992). The adventure features a trap that I'm having difficulty converting to 5e. It's ingenious but complicated, so I think I have to quote the full description, lightly edited for context. It's set up by two roguish antagonists, Psyche and Broc:

This room used to be the privy, and though the fixtures have been removed, the hole still exists. It's been covered with a piece of canvas about the same color as the floor, and the whole surface has been covered with dust using the cantrip spell from Psyche's ring. The canvas won't be spotted unless the floor is examined very carefully. Anyone approaching the book propped against the wall beyond the hole will step on the canvas and fall into the hole.

This trap is particularly devious because a second piece of canvas has been stretched across the hole a few feet below the first, and Psyche has laid out a portable hole on it. Anyone falling into the privy lands in the portable hole (which has been filled with water to 8' of its 10' depth).

Broc immediately yanks the whole business out into the crawlspace below the house, through the slit where they've cleared the mortar away from the brickwork walls of the privy hole (see diagram). He folds up the hole and rushes with it (so whoever's inside won't run out of air) to area 40 [the prison]. There he opens the portable hole out on the floor of a cell, then hurries out and locks the door.

As soon as the disoriented victim crawls out of the water -- which should be possible without help for just about anyone who doesn't panic, excepting a dwarf or halfling in full plate -- Brock grabs the edge of the portable hole and pulls it out through the bars before hurrying back to the crawlspace.

The victim's companions should believe he has dropped into the long-clean water of the real privy hole, 50' below -- and isn't surfacing.

I have the following issues:

1. I don't think 2e had a mechanic like Perception/Investigation for trap detection. Should I call for one of those rolls? Check passive perception? Roll it for the players secretly? Just assume they don't see it unless they specifically announce that they're looking? DC 20 seems appropriate if a roll is involved.

2. If one of the players gets locked up in the prison, they're basically stuck doing nothing until the other players manage to rescue their PC. That would be boring for the player. What can I do to keep them engaged while their PC is stuck separated from the rest of the party?

3. What if the imprisoned PC shouts for help? The prison is in the basement, sealed off by a trap door (on top) or a secret door (to one side), with a 3' crawlspace between the trap door and the bottom of the 1st floor. How would I set the Perception check for that? DC 30 minus the result of a CON check by the imprisoned PC maybe? With advantage because they can shout repeatedly.

The game is in 2 days. I'm hoping there will be some venerable grognard who can give me sage advice on this, maybe even someone who's run the adventure before.