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.
Earlier this year, I applied for a 10 year Cambodian driver's license, which is useful obviously for Cambodia as well as the rest of Asean and lasts twice as long as a Thai one. I did this because I spend a lot of time in that country and the police in Phnom Penh do hassle foreigners who don't possess a local license, regardless of Asean agreements and supposed recognition of IDPs.
However. I of course also maintain a Thai license as I consider myself a resident here so I renewed my licenses and IDPs all on the same day just over a week ago. Everything went smooth.