Will I be denied entry to Thailand with just a one-way ticket for my retirement visa process?

I buy a “throw away” ticket from airasia to Cambodia or Vietnam

I went through this process a few months ago and to save any worrying followed the advice and bought the cheapest flight online to a neighbouring country, it cost me about £20.

From Heathrow, Almost 100% it will be an issue from experience. Just get a cheap one way to anywhere is the easy solution.

I am doing similar with a one way ticket from Australia to Bangkok via Singapore with SCOOT Airlines. I spoke to a Scoot agent and she said that they take their ruling from the Immigration Consulate of Thailand. So it seems that it is dependent on the countries immigration policy NOT the airlines. Please correct me if you know otherwise.

This depends on your airline. Some let you fly without visa, Some don't

As you are entering Thailand Visa EXEMPT you will be stamped in for 45 days plus one 30 day extension of stay from local Thai immigration office for 1900 Thai baht. As you state that allows you time to take the steps to get the Non O Visa as retiree and the One year extension of stay as retired.

Your question on the need for another flight departing Thailand,

ASK your airline as they are the ones that are asking for it. Thai immigration will allow you entry... BUT some airlines want to see a ticket departing Thailand within the 45 Days STAMP which you get on arrival.

A simple solution, IF your AIRLINE wants to see a ticket departing Thailand...is to buy a cheap one way ticket to Phnom Penh...usually Less than US $40.

Hope you are successful with your plan...