After a brief detour into actual mapping, I've returned to where I belong. Here's more testing feedback:
LOST17 - Mephisto's Mausoleum
-The tally screen lists the level's name as "Mephistos Mausoleum" without an apostrophe. (It's correct on the automap.) Perhaps the big level name letters don't have apostrophe graphics?
-Yet again, the brightness values are identical to the PC version's and need to be darkened. (The exception to this are the various sectors affected by lines with linedef action 112--light change--applied, which cause the walkways around the cyberdemon to turn nearly pitch-black if the user's ambient light level is set low.) Coloration could stand to be spruced up as well. (Of course, these are all just optional cosmetic enhancements.)
-Bad misalignments exposed on the walls when the sector 36, 37, 38 or 39 platforms are lowered.
-The red sides of the teleporters in the sector 74 quadrants don't work: the teleporters have no destination targets. In the PC original, the red sides used to teleport the player into the middle of the room. Since that would now get a quick telefrag kill on the cyberdemon, the red sides of the teleporters should probably just be removed entirely.
-Sectors 44, 46, 49 & 50 should probably have ROK06 floor textures.
-The REDSK textures on lines 504 & 670 should probably be removed since the door can't even be opened from that side anyway.
-Nothing is tagged to make sector 126 lower anymore, meaning the sector 23 secret (invulnerability sphere) is inaccessible.
-Sectors 147 & 148 don't need to be marked as secrets. This is a minor bug from the original map.
-The monster-blocking flag on line 417 should probably be removed. It's a holdover from the original map. It had little impact on the original level, but now that monsters can easily walk into sector 9, it's easy for the player to exploit it. And once sector 22 opens, monsters can just walk around the line anyway, so the monster-blocking property is even more useless.
-For balance purposes, consider deleting the thing 143 megasphere. This will encourage the player to seek out the crypt in the northwest quadrant of the map so they can unlock the megaarmor and get easy access to the soul sphere in sector 128.
LOST18 - Black Tower
-Lines 243 & 589 can be removed. They no longer serve any purpose beyond causing a strange-looking strip of light on the floor. (Just make sure to close the hole they'll leave in sector 59.)
-It would probably be a good idea to change all the barons in sector 17 to face east. Currently, they have their back to the player when the player enters the room, making them easy targets.
-How about, instead of making sector 22 a teleporter, it instead turns into a perpetually lowering and rising lift when you click on line 978? (Actually, it'd have to be a walkover line, not a switch, since vanilla Doom didn't have switches that activated perpetually-moving floors. So maybe apply the walkover action to line 973 instead, and just remove the line 978 switch entirely.) Then the player will have to leap from sector 122, onto sector 17. This would better emulate the player having to ascend the staircase and cross the narrow bridge to reach sector 17 in the original map, and would also prevent the player from just telefragging the barons. This modification would also prevent a serious error, where the player can use the teleporter before opening sector 246, which renders the map unwinnable. (If you do make this alteration, you might want to move sector 122 northward so it's parallel with sector 242, replacing the thin bridge from the original map. Also, change sector 122's floor texture, and delete the thing 781 teleport destination, too.)
-I might replace the thing 73 nightmare demon with a chaingunner. At least it would have a marginal chance of attacking the player through the bars, like the arch-vile could in the original map. Similarly, replacing the nightmare demons in sector 135 with barons would be better equivalent to the arch-viles from the PC map.
-All the doors in the areas surrounded by the sector 66 ring are missing door tracks.
-If the "actors are infinitely tall" compatibility option is disabled, players can become permanently trapped in the sector tag 13 pits inside sectors 25 & 63. Even though this wouldn't be a concern in PSX Doom, for the sake of compatibility with modern GZDoom, it might be best to raise those pits up to floor level, and just have teleport destination spots in those sectors. Then when the player crosses line 150, a dummy sector opens and lets the monsters teleport into the squares where the pits used to be. (The ceilings of the tag 13 sectors should probably be raised to meet the height of their parent sectors, as well, since these pillars sticking out of the ceiling seemed to cause slowdown in DOSBox.)
-The thing 128, 133, 138 & 143 imps are unable to move or attack, due to the angle of the sectors they're standing in. I believe this is an issue in the original map as well. It may be best to just delete those imps entirely.
-Line 876 no longer has any sort of texture clue that suggests it might be a hidden door.
-The thing 263 hell knight is partly stuck in the walls and can't move. (This may vary depending on compatibility settings, but occurs with Doom Strict.)
-Since there's not enough "RAM" for a cyberdemon, at least add an extra baron to sector 140. There's room if you move the existing one to the side.
-Consider removing the thing 723 & 724 piles of gore. They serve no real purpose, and just clip into the walls and float oddly.
-The line 1378 linedef action, the line 1218 switch, and sector 207 are all now pointless. The switch used to reveal a hidden soul sphere right before the exit of the map. Now that the exit has been altered and the soul sphere removed, the switch does nothing. Either restore the secret soul sphere, or delete that sector/switch.
-Obviously, the exit room can't be exactly replicated from the original since it involved such a tall space, but why not have an ending room that at least resembles it somewhat? Make the room 256 units tall, dump the player in the lava, and have them fumble around, looking for a secret door as their legs burn off. The western wall is the true exit, but the wall lowers really slowly and they have to hope they survive long enough to escape, just like in the original map.
-Some feedback on lighting:
*This map shares the same brightness levels as the PC map. However, it's already fairly dark in many locations, so while I still think it should be darkened in some of the brighter spots, be cautious with it. There's also several light-changing lines late in the map that dim sectors, so be mindful of their settings as well.
*The sectors with key pickups need appropriately-colored lighting.
*Sectors 165, 177, 188, 191, 192, 198, 199 & 238 need their lightcolor set to F5D777.
*The sectors with BLOOD1 flats could probably use some red-tinged lighting.
*Maybe some greener lighting for sector 131 because of all the vines and green torches? Same for sector 163.
*Perhaps more blue-tinged lighting for the sectors with WATER01 flats? Also for sector 242 for the soul sphere.
*The sector 67 area and all the attached sectors have no colored lighting and should probably have some applied.
-Detail reduction ideas:
*You might want to tone down the number of strobe-light sectors--formerly the spiral staircase in the PC original--around sector 0. Having so many might have caused difficulties for the PlayStation, and since the staircase is gone anyway, they serve little function to gameplay.
*Sector 18 is divided up into four walls. I'd consider splitting the northeast and northwest walls off into new sectors, separate from sector 18, and removing the sector tags from them so they no longer lower when the line 1441 switch is pressed. (And also move/remove the thing 671 shotgun shells since they will no longer be accessible.)
*Consider greatly reducing the size of sector 22, as it doesn't offer a lot of gameplay benefit but allows the player to see a large amount of viewable area. Maybe just draw a line between vertices 544 & 547 and delete the remainder of the sector below it. (Just make sure to move the health pickups, the shotgun and as many enemies as you can into the remaining space. The thing 405 cacodemon can probably be moved down into sector 243. The deathmatch start can be moved at your discretion.)
*All the battlements in the sector 132 ring probably wouldn't have been nice for the PSX. They should probably be reduced in number.