@TheAtheistReverend - you asked "I want to do the same thing as you. Have you had any luck with finding answers to your questions?"
No - even though JHC said it would work. Being a car guy from the 60's and having done a lot of car re-wires and mods and working with computers and electronics all my life, and understanding how computers and solid state devices work in vehicles, I'm not convinced just cutting the brake/turn signal wires to the rear lights and then connecting the lights to the trailer harness circuit won't introduce other problems even though the both lamps will light for either braking or turn signal.
Unlike the 4 on high mod, which can be undone in less than a minute by just pulling the diodes, this mod requires cutting, soldering, etc and if it doesn't work right under ALL conditions, then we have a safety issue. Plus, having no load on the cut, and no longer used rear turn and brake signal wires, has to be detected by the turn signal module.
Since this is a much more involved mod (the way I want it to work), somewhat difficult to undo and can cause a possible safety issue (rear lights not working correctly) I decided to leave well enough alone. There is also the factor that if I sell my A/V (unlikely) the new owner will have no idea of the mod nor how this mod was made.
PS. I saw someone say old Avalanches never die, they just go faster. Unfortunately, in my case, it will rust out long before that.