Question #1: Is the redirect gear system I described above "below standards"?
Answer; There are not Written Standards for Rec Climbers. It is up to the Climber Desires and Responsabilty to be safe while climbing.

Question #2: Would a screwgate carabiner be suitable to build up the false crotch, as pictured on p. 63 of "The Tree Climbert's Companion, by J. Jepson, 2nd Ed., instead of the double-locking carabiner, pictured and documented there?

I use both types of Biners (Auto-Locker and Screwgates) on my climbing system and I consider it to be safe and appropriate for my taste.

Question #3: Has there been a study of, probably non_meta_llic, add-on components that would serve to make a strength-rated screwgate carabiner vibration-proof and reasonably un-thread-proof?

Answer: No,that I'm aware but I don't have all the answers anyway! (Vibration Proof? Never heard that properly closed Biners will open up if vibrated.)