As Steve said many insurance companies won’t cover you unless you are the owner of the bike. Travel insurance covers hospital bill. But you need to ask if they covers accidents cost by riding a bike. Then do they require you holds a license or permit.
Getting an IDP or Thai license is just for voiding pay fine when police stops you at a check point. For insurance purposes get a travel insurance which covers accidents riding a motorcycle. When you riding a rental bike there is a basic insurance coverage from Thai government but it does not offer much protection though.
before was 5 or 10 baht for regular Thai post. Take about 1 month. Now they charge 40 or 50 for express delivery. Around 1 week. If my memory is correct.
it’s not a 7 days visa. It doesn’t look good to have a 7 days denied stamp in your passport. If you really need more than 75 days but under 90 days. Just do a border run at the end of your 45 days visa exempt. That way you will have close to 90 days in Thailand. Without any bad record in your passport.