Im surprised a little, no one else than the immigration can provide a tm30. The hostel, the landlord, have to fill a paper/app/ go with you to immigration, yes, but the tm30 come from immigration.
not sure for Chinese. As said before, Chinese ppl are reqired to go to their country for visa. I know they can have 1 tourist visa for Thailand in Vietnam, but its an exception, and idk if ED can too. Asking embassy.
yes, and i think need go China. But, in Covid time, they introduce a fee of 10k thb in order to swap visa from inside the country, maybe still available.