exactly. This being his return trip after he spent 6 months in Thailand without a visa and then flew out for a day and back or something similar. There's a reason they don't include that stuff in their posts.
It's not that unusual. Embassies and immigration occasionally ask for proof of a fully paid reservation, flight or hotel, for various things. Just book a refundable ticket and be done.
Pretty obvious this person was already on thin ice to be asked for onward travel to begin with.
No. You need to show money in a Thai bank account in order to convert to a non-O visa, and you cannot open a bank account as a tourist. If you are going to Thailand in December, then apply for the non-O visa from the Thai embassy online before you travel. That may allow you to open the bank account on your trip.