1. They handle various loads and higher pressures better due to differences in the internal construction (more and heavier belts, steel versus nylon, etc.) and the tread patterns are optimized for the use (they're not made to cut a corner unless forced to by the tow vehicle, tread patterns are not like a car tire at all). They're also generally a low rolling resistance compound and pattern.
2. No clue, I've went with whatever brand the reputable local tire guy recommends.
3. No clue, I've never had an issue with a trailer tire besides the random nail making a slow leak.
4. Sorry, no help there either.
I will look at my trailers and see what's on them and report back.
Subscribed! Good questions.