correct, if he didn't have a non-immigrant visa then he would be asked, that being said I know of someone who has a work permit who the airline in PH refused to let board until she bought a flight out of TH at check in. In that scenario, the check in in PH were wrong and she should've stood her ground and refused.
they're ultimately responsible should immigration in Thailand stop you from entering. Some airlines are stricter with onward departures than others but just saying you're leaving overland might not work.
The person next to me at check in at Heathrow was asked to show a flight leaving Thailand as he had only booked one way with Emirates. Don't know how it got resolved as I finished checking in before he did.
You will only need the TM30 from the hotel you're staying at when you go to get your extension. On the basis you're not doing the extension whilst staying at your current hotel I wouldn't worry about it.