A Shadow Over Hackdirt is almost complete, but I'm afraid to talk to Seed-Neeus. My memory tells me that there is a bug with her. She will raise my Mercantile, but from that point after Mercantile cannot be raised anymore. Checking the wiki shows none of this. Was this fixed in the last Official Oblivion Patch? The fix from tesnexus is gone.

http://www.tesnexus.com/downloads/file.php?id=5296
http://www.tesnexus.com/downloads/file.php?id=27710

I don't recall that ever happening in my games. And I always to that quest. *shrugs* But I do always also have the Unofficial Patch running...so maybe that's why I've never encountered it.

Create a save before talking to her. Reload if Bad Things Occur.

I think you may be getting confused with an old issue, where if a skill was increased by scipt rather than using it, you then have to use it enough to cause that increase before it would start counting again (since mercantile can be quite slow this could take ages). this issue was fixed by official patch 1.1.511 so if you have the ;atest patch you are good to go.

Indeed. I have 1.2.0416 and no problem at all. In fact, a few days ago I lowered my character′s merc skill because I considered it to be too high