You can include a brief cover letter explaining that you have been traveling for the last little while and intend to restart work, and maybe show your income pre-traveling. Suggest you also provide a portfolio and maybe some of your previous work contracts
It's the airline that can refuse you boarding if you don't have a long term visa and plan to enter visa exempt. So, contact your airline and ask. Immigration in Thailand won't question you on this if you're an occasional visitor.
the way you relay your situation is ambiguous. You say you're already living in Bangkok, you say you're on a tourist visa, but that you are in America. You still haven't answered what was your proposed entry date into Thailand on your application? Processing times are reset when further documents are requested and received.
on the application it asks for proposed your arrival date and method. I'm confused, did you get your visa and return to Thailand using it, or did you return to Thailand before it was issued?