Who will check your onward travel? In my experience, the airline flying you to Thailand, and possibly Thai immigration.
Regarding your application for a 60-day tourist visa, you should check with that embassy about what they need and how long it takes: there's a lot of variation between embassies.
Sounds like an allergic reaction - a dermatologist I was talking to reckoned everyone has an allergy threshold, and a increase in temperature, humidity, maybe even cleaning products can tip one over into irritation.
Like others have said, you probably should try to visit a dermatologist.
We just flew in two days ago with TR visas (from Wellington, NZ Embassy) and were asked one question, "How long are you planning on staying?" Given we'd supplied LOTS of info to get the TR visa, (bank statement, rental agreement, et al) I'd have been surprised if they'd wanted all that again.
I've applied for a TR visa within NZ. Here, I have to provide the application physically to the embassy (no online option) and they are stringent: return flights required within 60 days, and a bunch of other things around accomodation.