2. Yes - but Vietnam generally requires a tour. Laos, Cambodia, Malaysia and Singapore are the destinations for which you have free reign on a Thai registered bike or car.
I can't imagine they wouldn't do an eyesight test in Krabi though as that is literally done everywhere and I've done it in multiple locations around the country over the years. All 4 tests take a combined 5 minutes or less.
Same elsewhere. I got my 2 licenses renewed in an hour and my 3 IDPs took another hour but only because of having to get more photos and photocopies done.