If Trailex hasn't had a problem, then you can't argue with their 8" tires on their lightweight trailers. I'm sure they work fine, and I don't think people should be concerned about them, as Trailex seems to be a quality company that will take care of their customers.
I disagree, however, with the bounce situation, but haven't run an experiment. The theory that a small tire vs. a large tire is like a "basketball effect" doesn't make sense scientifically because the wheel isn't being bounced like a basketball, it's being rolled towards bumps. Roll a tennis ball towards a 2x4 and what happens? Now roll a basketball, now roll a large exercise ball. The rolling resistance of the larger ball, (to an extent) is also more helpful when going over bumps.
If Trailex is experiencing excessive bouncing with larger tires, there seems to be something else at play. I can't figure out what that is, but the larger tire, if properly inflated, should do a better job, not a worse job with trailers, even if they are lightweight.
3. The 12" tire will require you to back further down the ramp. On shallow ramps, it would probably cause the tow vehicle's rear tires to even go into the water. If you can lower the bunks on the trailer to make up for the taller profile tire, that might help.
This isn't a problem with lightweight boats like kayaks, Pro Anglers, or even my Bravo. I have submersible lights on my trailer, but almost never back them into the water. Driving up to the water's edge, or a bit into the water, is all that's necessary to get a lightweight boat off the trailer. These boats float in mere inches of water, you don't need the assistance of having the boat float, to get it off the trailer, you can simply slide it off the back end, and let it pivot towards the water. It works great.