I think T.L. was pulling in your favour, as fan works of copyrighted characters are technically illegal, so it really becomes a whole "glass houses" thing. At least two people here are on your side.
When it comes to modifying unauthorized doujin to tell a different story, I say "go for it".
If worse comes to worst, discovery would be a blast.
Basically, the "original" artist would be in a position where there artwork is readily considered illegal, most likely under the DMCA or Berne Convention depending on where the server's located, while attempting to mount a legal challenge in a court of law.
I wouldn't worry about it.