1. Depends on the trailers GAWR and GVWR and actual load on the axles. All need to be considered.
2. no but much depends on answers to #1 answers.
3. IMO as long as the larger tire has clearance in the wheel well a next size up can be a good upgrade.
4. The Greenball have the most tire complaints on NHTSA/dot.gov website.
..Carlisle makes several line of trailer tires. The only one I would choose is their new Carlisle HD.
..Maxxis has a checkered history of failures over the years and still 65 mph rated. Most other are 75-81 mph rated. Speed rating are more to do with a tire ability to deal with heat as our load carrying tires on our trailer are subjected to.
...Goodyear Endurance is a new gen ST tire out about two years now. So far very few if any reliability complaints and a good tire for the money.
If your trailers sits 11 months out of a year and makes short local runs then cheap tires may be for you. If the trailer is on the highway at interstate speed then IMO a better tire is worth the money.
Were not required by any law to use the same size tire as the trailer mfg picked. Your only requirement when replacement comes around is...... the sum of the maximum load ratings of the tires fitted to an axle shall be not less than the gross axle weight rating (GAWR) of the axle system as specified on the vehicle's certification label.