1. Pretty sure ToM was one of the last books for 3.5 to come out (along with ToB).
  2. This means that if a theurge class wasn't in ToM to combine Warlock with Shadowcaster, then it probably doesn't specifically exist.
  3. Since you don't want ideas on how to get into classes that don't specifically advance features of each class, I'm pretty sure you're gonna be up a relatively well known creek, without the proverbial paddle.
  4. If you did want to join an existing theurge class, the best way would probably to utilize the text pertaining to Shadowcaster that says they can qualify for arcane or divine spellcasting requirements, and use the Eldritch Theurge PrC to advance them both.
  5. If you don't like that option, then you should probably go to the Homebrew sub-forum and ask those wonderful people over there, since this sub-forum doesn't really deal with Homebrew, other than "yeah, that sounds like it could work with a little permission from your DM, ask him/her".