1. Same size tires on the same axle however tread design in more important. Tires with different tread design (all season vs a all terrain type tread) can have different braking performance that show up on wet pavement. I would use the same/similar tread design on the same axle. ST tires have similar tread designs so it shouldn't matter.
2. Can't answer that one for ya'.
3. Take the lug nut to a auto parts store or a trailer parts house and get the size required.
4. I have over a million miles towing trailers up to 22k GVWR mostly commercially so I do all my own flat tire changing as I have all the necessary equipment and have done it dozens of times in all types of weather and road conditions.
5. New trailer or new wheels just installed or a wheel after having a flat should be checked for proper torque within a couple of hundred miles of last service.
I use a tire thumper and a visual at each stop. Each tire should sound/look the same. If one isn't I check the pressure.
Your using a ST tire so I would have a TPMS installed. If you don't have the equipment or proper know how for changing a flat tire then its time for a roadside service plan or... get the proper equipment and practice at home. On the road can be a humbling frustrating experience to learn how its done...... safely.