In the southeast of secret #1's entrance is a cage with a megasphere and a revenant who starts moving around as soon as he hears you making attack noises. He certainly will ride the elevator (sector 99) down, leaving it and trapping himself behind a lava pit. After pushing the switch in the west of the great hall with the invulnerability on the large crucifix, a door opens in the southeast. Assumed you did not kill the revenant yet, lower the elevator here and instantly step on it to ride it down, causing the trapped revenant to fire at the hell knight (baron of hell on hard skills) in the lava pit. Enjoy them infighting and kill the weakened survivor. Lower the elevator again, take an inrun and jump over the lavas pit: the very eastern wall can (not from the pit's ground) be walked through. Take the elevator you find here to access the dead revenant's cage, containing a megasphere.
In the sludge-filled area in the east of the map, there is an out-of-place toxic waterfall behind a long torch. This waterfall can be walked through, revealing an alcove (sector 202) with a soul sphere.
After grabbing the yellow skull key and dealing with the trap sprung at the red key entrance as a result, you see a metallic bar at the north. (Walking towards it over the now lowered walls (look at the automap) lets an arch-vile teleport into the center of the spider masterminds' arena.) Open it to find seven energy cells, seven sets of shells, seven rockets, and five medikits (sector 201).
The items placed at the spawn points of coop players #2, #3 and #4 cannot be collected in single-player mode, but are not flagged as multiplayer only: Three plasma guns (things #9, #10, #11), three bulk cells (things #13, #14, #15) and three medikits (things #17, #18, #19).