Immigration. Either at the airport or an immigration office.

If you want to leave the country during that 90 days and keep your current permission of stay date, then yes, you will need a re entry permit.

Make sure you put the visa number in the space provided. Hopefully that will make the IO check for the visa and stamp you in for the correct amount of days. Sometimes they make mistakes :)

For the sake of $30 each they would avoid all the hassle that you already know they will get when boarding the plane. Either get a tourist visa before they set off, or buy a cheap one way ticket out of Thailand before 30 days after they arrive that they can throw away.

Its valid for 90 days which means you get 90 days in which to use it. When you do use it you get stamped in for 60 days.

I can’t see it.

The arrival card is quite possibly the least significant form you will ever complete in Thailand. There isn't even a section that asks how long you're staying for.

Post a photo of it with your personal details blanked out?

I’m in the process of helping someone get uk documents legalised in this way at the moment. Definitely difficult to do if you’re not in the uk. The Thai embassy wants payment in cash or postal orders which would be a nightmare to organise from outside the country.

Whilst the lovely people in the London embassy are indeed experts at not being clear, it is worth remembering that embassies and consulates that issue visas are not anything to do with the immigration department and often don’t know how those visas can be used at the various Thai borders.

When they gave general advice of the entry stamp being ‘up to’ one year, they were most likely including the non imm O-A in their all encompassing statement.

Best advice I’ve heard about embassies and consulates is to get visas from them and ignore EVERYTHING they say about using them.