Landing agreements supersede dumping, denied boarding, etc
Bumped can be used to describe any number of reasons that results in 'denied boarding'.
Your phrase: "let them find their own way home" is EXACTLY what will not happen if Ryanair is required to meet it's obligations. What usually happens is they use a different carrier - Cathay Pacific terminated a pilot who happened to be in the U.S. on lay over and flew him back on a competitors aircraft.
And Ryanair wouldn't get away with dumping across the nearest border, either, as present travel tracking can be used to prove a passengers intent.
Single, as in no return flights, can be purchased for most destinations. the right terminology in 'onward' - countries can require 'onward' rather than 'return' ticketing.
I live in a country that requires proof of onward travel, so the difference is familiar to me. Onward travel can be by any means, as well, including road, rail sea or air.
Besides, proof of onward travel is very ephemeral with e-ticketing, since changes/cancellations can be made by e-mail these days. Whenever I fly through a drug hub, such as Bangkok, I always depart on the passport of entry to Thailand (a necessity) but land at the next destination on one of my other passports - removing any thought I might be carrying drugs. Never been challenged on this, either.
As for seeking assistance from diplomatic sources, most countries reps aren't worth even contacting for assistance for their first line is that 'We can't help you with legal matters, only ensure that the host countries legal procedures are followed'.