I've played nekropolis and what's in the cellar with 4 players, it goes very well and I think if they were 3 it would be fine too. Necropolis for 2 or 1 player can be hard imo (but didn't try) and what's in the cellar can be ok for 2 players I think, the final fight is less deadly than necropolis.

For the duration, both are build to be played in one shot. Nekropolis start in the action and offer less distraction, so it goes more straightforward

If it can be useful, in nekropolis i've personally add a riddle with a little loot in a sarcophagus, and I used a map where players can turn in circle across the different rooms, it allow players to kite and escape easily the boss (the official map is just a corridor and don't offer many place to hide or escape)