It may be or it may not be. It depends on the immigration officer who sees you and whether he or she thinks you're trying to live in Thailand on a visa exemption.
you used to be able to do it on their website, I did it on my laptop. I'm assuming you still can but I haven't done one for nearly a year now so don't know for certain if that option is still available.
No one can give an accurate answer as it very much depends whether the immigration officer upon entry to Thailand decides if you're a genuine tourist or not. Chances are you will probably be fine with your plan.