How strict is Thailand in checking for a return flight for a 30-day visa on arrival, and can I extend this visa easily?

I bought a throw away ticket to Singapore. The airline check-in counter only wants to see onward flight, not proof you can enter the country you have a ticket to

Visa on arrival (VOA: buying a visa with fee upon entry Thailand and will be stamped for 15days) and visa-exempt entry (enter Thailand without a visa, stamped for 30days, and don't have to pay for the fee under the visa-exempt scheme) are totally different programs. Nationalities that are eligible for Visa-exempt entry cannot apply for VOA. Vice versa. "EU countries" include both (and other) types of nationalities.

As long as you can get a Thai Pass, Thai immigration would not care if you have a return flight or not. But you may be asked for an outbound ticket at airline check-in.

IF you are eligible for visa-exempt entry, you can apply for a 30days tourism extension easily.

IF you have the nationality of VOA countries, tourism extension is not available for you.

Immigration doesn't really care, it's mostly the airlines that check and can refuse you boarding. yes you can extend once for 30 days

I have only had the airlines look for my departure ticket. Historically, I have just purchased a throw away ticket from airasia. Cambodia is opening to flights. As is Malaysia. No one ever checked my ability to take the flight