Yes you can, they may ask you about it but not an issue. Last time i entered was tourist visa was marriage visa before. They asked me and i said need time to reapply no issue. Still undecided whether or not to do DTV.
Head down to Bangkok pantip plaza or fortune town. But most towns have got somewhere why do you need to import them if low end and pay taxes. Although my wife did order a laptop from Amazon in the states and with tax was still cheaper than here.
No you cannot, i tried that there many years ago they made me go back to Malaysia border and get stamped in with passport I left Thailand and then come back. Flying is only way to change passports and always found KL or Singapore the easiest.