Would this service use Platform 3?
IIRC, the illustrative platform occupancy chart produced by Tees Valley CA states exactly that.
Actually reading a newspaper article from 2019 (as you do), Ben Houchen said there's an alternative plan until platform 3 gets the go ahead of just operating two or 3 trains a day. He also mentions a reversing point I didn't think of which is at South Bank on the freight lines and coming on and off at South Bank Junction. Its a possibility if its a 9 car working it, it'll need a safe walking route fitted (to be honest thats quite cheap) if they plan to use 10 car (2x5 units)
South Bank seems like the most logical place to send an empty train if you can't turn it around on a station platform. There's two tracks next to the passenger line that don't seem to have a lot of use, so that looks easier than any other the other mooted options.