You get 30 days when you reenter Thailand (just went on a Burmese whiskey run yesterday with a few friends). Also pick up some Burmese whisky in tachileik too.
Yes you can. Just need the resident certificate from immigration or your embassy, signed copies of the sellers documents, greenbook, and photocopies of your passport photo page, entry stamp and departure card.
Immigration wont ask unless you have a long history of staying on visa exempt entries. Usually 3 in a row with extensions, then they start asking you about why you're there (and generally its to make sure you are not working illegally)