wrong , you can carry whatever you want but you need a export and import declaration for some amounts. For in Thailand that is over 20.000 USD or equivalent in other currency. Check your country of departure for limits on exports.
No . You have a visa the first year. After that you have an extension of stay based on non OA. And if you want to keep that extension alive you need a reentry for that.
no. If he still has a multiple visa it will just start another year. If he comes back after the visa has expired its 60 days on extemption and he can convert that to a non o retirement. If he has his papers in order. I killed my own non oa that way last year myself. Easy process
Yes. If you getting non o visa. You need passport and a resident permit from immigration. Ask your temporary hotel for the paperwork needed. This is the easiest thing you will ever do in Thailand...