with more than 800,000.- THB in your bank account, you should experience no difficulties to get the bank letter for two months seasoning in order to prove the financial requirements
doesn't the bank print "condensed transfers" in and out of the account? As a summary? your problem is: for the applications you need a bank proof of 12 months, month for month, showing incoming from abroad minimum of 65,000.- THB. If you miss out only ONE month, your application won't go through. You would be better of with the 800,000.- THB in your account, it would be as easy as it is peasy
the airlines bother, simply because if immigration refuses your entry, the airline is, by law (!) responsible to carry you back to the port of origin at their own expenses. And the airline will get fined by the IATA for 10.000.- US$
Savannakhet changed their requirements as of lately. You now need the 400,000.- THB financial proof for the multi entry Non-Imm-O family. However you can get the 90-days single entry Non-imm-O Family visa without the financial proof
if you fly Finnair out of Helsinki, they will not ask anybody for an onward travel proof - until now! My friend flew out from there without a visa 5 weeks ago. However they might do so in the near future, should they ever have to return a passenger whose entry into Thailand got refused, because it will cost them 10.000.- US Dollar in fines
he can't apply for the "change of visa type" from a tourist visa to a 90 days Non-Imm-O visa, because for this process, one needs to have a minimum of 15 days left on his stay permit. Which he does not have any more and he also already blew the 30 days extension. He might be able to do the "change" from out of a 60-days "family visit extension". This he needs to inquire at his immigration office. In case they deny, all that is left is a new 60 days tourist visa in one of the neighbouring countries, or the 90 days Non-Imm-O in Savannakhet
Thai Immigration actually is not interested in your return ticket or flight out of Thailand, when you arrive without a visa and enter visa-exempt. Only if your passport shows an intensive travel history to Thailand, you might get pulled aside and asked.
In the majority of all cases, it is the airline which wants to see a proof of travel out of Thailand within the 30 days period you will get stamped in on a visa-exempt entry
You can solve this issue with a cheap “real” one-way flight ticket. i.e. Air Asia from Hat Yai to Kuala Lumpur for like 32.- US Dollar
You can try your luck with one of the “fake” onwardtickets, but be prepared that they might not be accepted
Some people claim that a bus- or train ticket out of Thailand, or a hotel booking in the neighbor country was enough of a proof, but again be prepared that the airline will only accept a “real” flight ticket
There is NO GUARANTEE that your experience will be like some people in the FB groups will tell you, claiming that they have never been asked for an onward travel proof.