Stolen Earth doesn't really help your argument out any, since that wasn't really an actual regeneration. That is, they weren't switching actors at that point. RTD does not have the authority to tell Chibnall to leave the regeneration out of the finale now that it's been filmed. That's set in stone, RTD has to work around what Chibnall has written. Indeed, based on tradition (at least in the modern era), the preceding showrunner checks with the succeeding showrunner to see if they want the new Doctor in the finale and don't write the regeneration until after they have their successors consent. I don't know if the timing works out this time around, IE if Chibnall knew RTD was taking over when he wrote the finale, but either way, RTD has to comply with what's already been written. Either Chibnall did check with RTD if he wanted the new Doctor in the finale before writing and got his permission, or Chibnall, not knowing who the next showrunner would be, Chibnall wrote a regeneration scene anyway and the new showrunner would have to work around that.And RTD has form for leaving a regeneration as a cliffhanger (Stolen Earth) so if he said to Chibbs, “Chris, you’re not going to have a second half of the regeneration. I’m going to start 14 in media res.” that’s what happens.
Besides, RTD realizes the importance of showing the new Doctor in the preceding Doctor's finale. In The Writer's Tale he talks how he spent an entire week before he had a meeting with Moffat preparing arguments to convince him the Eleventh Doctor had to be featured in TEOT, only to find out when he met with Moffat that he had spent the preceding week preparing arguments to convince RTD of the same thing. I don't see why RTD would feel different this time around.