Sorry visa exempt.

Maybe just go back into Thailand on a Visa exempt. Looks like your options are very limited

I'm doing it next month, getting a 11.20pm flight to KL from Don Muang, staying in a airport transit hotel for 6 hours and returning at 8.00AM in the morning. Doesn't answer your question but I know you can stay in the airport

Thought

I though Phnom Phen was always a place to avoid for visa's.. for the reason you state about proof's of.. I have always bounced fro KL to Vientiane to Savanaket. Seems to be always fine for T-visas. Seen rejections for other types of visas though

Mate if she has them requirement there will be no problem at all. My gf has been back with me the past 2 Christmas's. Has a secure job but no property. Easy to do it yourself. All info is online. My gf said a back log of payslips and bank statements is one of the key things/ is she financially stable to afford the trip. Also on her application she put we are staying at my parents house (not in a hotel) and provided my mum's details. Sailed through both times.. only waited 7 days. Don't stress and just make sure she has all the required documents