The airline shouldn't have a problem. When I came this way I didn't have a return date because I was moving here temporarily and had to change my Visa once here. The airline made me sign a paper saying that they did explain to me that I might get turned away and not let into Thailand but I have not had any problems. Done that a couple times. Wasn't even questioned at the Thai border.

Not from my experience. But they do take a copy of your passport