UPDATE: got hold of the department head today and had quite a constructive discussion for 17 minutes about the issue. I sent her the Home Office carrier communication (again) and she appeared understanding of my concern, reassuring me she'd properly look into it and get back to me addressing whether or not to require proof of UK residence status going forward, AND if so, how to best supply it as a passenger.

Based on the conversation, I see three issues within Ryanair:
* Although the head of the Immigration Department is knowledgable enough on the subject at hand, not everyone in that department is. Someone there recently told a Swedish handling agent that EEA/Swiss ID cards are never accepted for travel to the UK, but the head confirmed this is NOT their policy at all, with anyone with UK residence status being OK, which also matches their T&C (though again, only EUSS residence status actually qualifies)
* The Immigration Department doesn't seem to understand how worthless the customer service (CS) is. She told me passengers are encouraged to go through CS, and I heavily emphasised I'd tried that route for many months, yet never received any response of substance/value from them.
* CS apparently never involves the Immigration Department unless it's an EC261 claim where a passenger has actually been denied boarding, even when I explicitly ask them to.

So in short, things are moving at last. Eagerly awaiting her information