I'm talking about SE Asia in particular.

And, like it or not, your potential problem is at the check-in desk, not arrival immigration.

Online check-in is a fine idea, but it doesn't always operate as smoothly as you might think. Best not to be smug about it. A one-way ticket, indeed any ticket, can land you with an "acknowledgement of check-in", with a requirement to pick up a boarding pass, with a document check, at the airport.

Again, airlines are simply protecting themselves from financial penalties by doing these checks. They might want to see proof of a visa, or eligibility for VOA/entry, that you have a passport with the required number of empty pages for stamping, a passport with the required validity, as well as proof of onward travel. If you get turned away at the arrival airport, the carrier can face fines for NOT carrying out document checks.

I have never, ever been asked for proof of onward travel at any arrival immigration desk, anywhere in the world. But travelling on one-way tickets in Asia I have very often, perhaps always, been asked for my onward itinerary: usually a PNR/flight numbers suffice. By now i am prepared for it, which is why I am hazy about "often" or "always".

I learned my lesson at Chennai (OK, not SE Asia!), on a one-way award ticket to Doha: it got quite sticky because i couldn't call up details of my separate Doha London flight on my phone.

You asked, but you seem to want a sunny, positive reply Your plans could work out fine, but there is a very real chance you will run into problems, somewhere: whether or not these can be resolved easily is in the laps of the gods. But it wouldn't take much effort, or expense, to avoid any chance of this happening, by providing yourself with onward travel documentation.