What do I need to show when I arrive in Thailand regarding vaccination proof and accommodation booking?

Passport and boarding pass only needed to enter assuming you’re visa exempt.

Depends how you’re entering. If you have a visa then almost certainly all that you’ll require is vaccination proof. Even if you don’t have a visa it’ll be very unlikely that they’d require accommodation proof. Unless you’ve got a long history of entries in to Thailand or a history of covid stamps. By law it’s needed. Very rarely checked on entry - unless they suspect that you’re trying to buck the system with a lot of entries.

Strictly speaking, letter of the law... Yes.