As other commenters note, you've got up to seven days after the 90-day report is due and I can vouch for that, I always do mine a few days late as I do it at the end of every three months.
Surprised you're only getting 60-day extensions on an ED, though. It used to be 90.
Both Roojai and DirectAsia offer the full vehicle or nominated drivers option, nominated drivers bringing the price down a bit, at least the last time I got quotes from them.
It depends - typically it's the vehicle but some policies here also have the option of nominated drivers/ drivers over a certain age etc. for lower rates.
You can do either the extension at immigration or apply for the SETV before you leave. However, I'd recommend getting a tourist visa and there is a reason why - the dates you are traveling.
December is high season and particularly with all the public holidays, Immigration offices will be super busy in the more popular locations. You can always risk Immigration but if you're somewhere like Bangkok, you going to lose the better part of a day getting there, getting the extension then returning.
Yes - except for when it isn't. Happened to me once - boarding gate at CNX, showed my DL and boarding pass and they insisted I show them my passport. Other times never had a problem.