Okay, so the update:
1. The "short" edge distance on the stiffener is not a problem.
2. Countersinks for 1/8" are acceptable, since it's just a single location.
3. After drilling the aftmost hole that attaches the F-783B cover support rib, I found that this hole is even worse, this time it's definitely "out of spec" with the ~0.190" edge distance. Also, according to Van's support, not a problem.
So I think I'll go the simplest path and do it the way instructions say, but with a -4 rivet.
This is the edge distance from the third item above: