It sounds to me as if something is really hosed in TS12. I would just import the route and get the missing dependencies within T:ANE, which will be the ones you are looking for.
I just went through this myself recently and this is what I did.
I imported the route into T:ANE.
I then downloaded any missing things that could be on the DLS, which took a few minutes.
Once that process was done, I then checked the missing dependencies and the number dropped from the 3000 range to about 226!
These 226 were a combination of my own custom assets and third-party assets.
In T:ANE CM, I sorted on status, which put all the unknown assets at the top of the screen.
I highlighted these assets and pressed CTRL+C
I then pasted the list into a text file and did some editing.
In the text file I replaced all the > with >,
I then copied this list again, I did it in chunks this time because it was easier to handle, and pasted that in the Asset Kuid filter in TS12 CM.
This would bring up the assets by Kuid, which I opened for edit and imported these into T:ANE. When T:ANE was finished importing, I then reverted these in TS12 CM and repeated the process again on the next batch. The importing process took me about 20 minutes to go through.
When I got all that was listed, I then checked for missing assets again in T:ANE, which showed some dependencies of dependencies I needed to track down. I repeated all the steps above including checking the DLS, and with this being only a mere handful, I was able to resolve all the dependencies.
If you can open the route up in TS12 first, I recommend doing that to replace all the Speed Trees if you have used any with something else before importing into T:ANE. That will save doing the tree hunt in T:ANE because the older trees won't work.
