Thailand don't care about your ticket status its mainly here in Australia that they could stop you boarding if its only a one way ticket, it all depends on the airline and the person you get booking you in
buy from where Bangkok or Singapore, I have always been denied buying duty free alcohol in Bangkok if I'm transiting through Singapore so maybe the rules have changed I don't know
I have always been denied buying duty free if I was transiting through Singapore from Bangkok back home to Australia, as soon as they check my ticket I'm told it can't be done
It all depends , I can not buy duty free in Thailand on my way home as I transit through Singapore so not sure how it works in your situation, the duty free shop you buy from will be the best people to ask