In the Suran Underground mod, I'm trying to do the quest where I deliver a message to Talev, who has a home in the Redoran Underworks. I searched the entire Redoran underworks without finding him, or the door to his house. So I went in the CS, found out that his cell is called Talev's Abode, then went back in the game and used a console command to put myself in that cell. I figured I'd walk out the door, see where in the Underworks it was, then restart the game and find it the normal way.

When I stepped out the door, I found myself in an "alternate" Redoran Underworks. It was exactly like the one I'd already explored, except it had the door to Talev's Abode where the area I'd explored had a blank wall, and it was completely blacked out on the map, as if I hadn't explored any of it yet. I went to the nearest ladder, climbed back up to the Canolworks, went immediately back down through the same trap door I just came up--and found myself back in the original Underworks, the one I'd completely explored which had no door to Talev's.

Any ideas as to what's going on? I guess I could just console into Talev's and get on with the game, but I'd rather know why this is happening.

This is speculation. It sounds like one of the mods you have (most likely Suran Underworld) has renamed the Redoran Underworks cell in some way. Another likely suspect that you may be using would be Vivec Redesigned (I may have the name wrong here). I suspect that you have a mod that has copied the underworks cell and renamed it (such as changing the cell from "Vivec, Redoran Underworks" to "Vivec Redoran Underworks"). Since this mod's cell name is not the same as what's in the morrowind.esm master file it loads both cells. You can identify which mod is causing the problem by using the console to get to the cell that has the door as you did before and using the ori command on any objects in that underworks. Then exit and enter the other underworks and ori on several objects to see which mod created that cell. One of the two will have most of the objects added by Morrowind.esm the other will have most objects added by the mod which is causing your problem.

You nailed it! The culprit is LGNPC Vivec Redoran. The name change was even more subtle than you suggested: they just put an extra space after the comma, turning it into "Vivec, Redoran Underworks".