I think this site should answer several of your questions.
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I found ThaiPass to be a huge pain in the ass. Other group members had better experiences, seems to be a crap-shoot.
I would recommend asking the group for a recommendation of how to book and with who to book your hotel and tests. I think it maybe helpful to use a good hotel. Another member said they used a service to handle ThaiPass, if I had to do it again I would strongly consider paying a little extra to somebody to deal with the stress.
Best of luck