That rather depends on their nationality and on the type of visa they use to enter Thailand.

It's a bit of a "piece of string" question. Days if they're quiet, weeks if they're busy.

I'm guessing you mean a 30 day visa exempt? They go to the local immigration office and pay 1900 baht for a 30 day extension.

Depends on your ticket.

I'm assuming that things are the same as the last time I through checked (5 years ago).

If it's one ticket booked through, then the bags will also be checked through and you should clear at Samui.

If they're separate tickets, or the onward airline doesn't have an interline agreement, then you'll pick up your bags and do immigration in Bkk- then recheck your bags.

I don't believe they can.

I won't comment on this individual case as I'm not the expert. But when someone says "it's fine", I want their name on official headed paper with an official stamp.

You don't pay for extensions at a thai embassy as there are NO thai embassies in thailand. You pay it at the local immigration office wherever you are staying.

I think I'm right in saying that (generally) in and out on same day is fine (though I think that a few land borders insist on at least 1 night spent out). I'll doubtless be corrected if I'm wrong!

If the visa is still valid and is multiple entry, then so long as you carry the old passport complete with visa it should be fine.

Re-entry permits have to be purchased BEFORE leaving. You have to start again I think.