Buy at duty free, transfer into Ice Oi Ichi Tea bottle bought from the pharmacy. (Comes with free ice tea).
Tell shop attendant you're only going as far as your first flight because otherwise they're going to spend 20 minutes figuring out if duty free items are allowed to transfer.
* You need a copy of your passport to buy a ticket at the counter. You don't need your actual passport.
* You don't need your passport to board the train.
* Although in Thai law you should always have ID with you. In practice this usually (but not always) means that also a copy is okay, or a Thai driver's license is ok, etc.
Yes; doesn't matter if it's your first, second or final extension. It will all get auto-extended to April 30 first, and then later we'll hear what's up after that.