So the adapter plate and Jeep shaft swap will likely work very well.
Here are some of my measurements tonight.
All of this pertains to an unmodified Jeep 10 spline input shaft that's ALREADY INSTALLED in a T5.
The pilot tip is 7 + 1/8" from the front of the case.
The pilot tip is 1.5" long.
The input shaft bearing retainer sleeve extends 4.5" from the front of the case. (An S10 version that has NOT been shortened)
The Jeep shaft splined section is 2 + 3/8". (same as your factory shaft splines)
So your calculations seem correct. You will need the pilot tip to be 7 + 3/8" from the front of the case. The Jeep shaft is 1/4" too short but it may still end up inside the pilot bushing. Using an extended bushing might be a good idea.
So the Jeep splines extend out 5 + 5/8" from the front of the T5 case but when you use the adapter plate, they will only extend 5" beyond the front of the plate. Your factory splines extend 5.5". So we're 1/2" short there, but you might be OK. It all depends how much of the Jeep splines actually fit inside the clutch disc hub once installed.
The S10 bearing retainer should be plenty long enough. It will still mount to the T5 case and then sit inside a recessed area in the adapter plate. It will thus lose 5/8" of functional length. You calculated 3 + 3/4" and I'd save that cut for last. (4.5" minus 5/8" = 3 + 7/8" - so it is plenty long enough but may only need a 3/4" trim).
