Dan Goodwin 20k THB cash or same in other currency, proof of accommodation and flights out of Thailand. These may NOT be asked for, but any or all can be asked for at any time, especially if the IO wants to question the reason for your entry into Thailand.
It will depend on the IO on the day as there is no rule or limit to entries by air on Visa exemptions or tourist visas. Have 3 proofs as Robert recommended.
Getting a new passport to 'erase' any previous entries went out the window the moment they out everything online. 6 month visa is a good recommendation for a long stay and mutiple entries.
A Yes. B Yes. C Yes. Everything is online. New passport has zero affect on entry into Thailand. The only affect it has is for applying for visas in neighbouring countries as they do not have an online database.
Supposed to be within 24 hours of arriving back in the province after entering Thailand. They usually allow a few days though. You really only need it if you will do an extension of some kind. If not then it doesn't really matter.