I have come to the conclusion that they are not going to do it. Your post helped me to understand why. You said fixing the problem is not cookie cutter. Having written code I started thinking. Pop3 worked in IOS7, so how hard can it be to pull or tweak that code module. Then I asked what major code change happened after IOS 7 that would require rewriting the email section?
So...here is a theory. The one big change after 7 has been how you do upgrades. With 7 and previous upgrades the process could be a pain because you pretty much had to wipe your machine to have room for the install then put everything back. That was an IOS tablet/phone problem that was costing them. As folks have added more apps and the apps have gotten bigger and more complex, upgrades became more of a pain. Apple's first solution was ICloud, but folks aren't using it to the degree they thought. In fact the whole theory of cloud-based computing is starting to resemble a dead end. So with IOS 8+ Apple came up with the brilliant, but flawed idea of being able to make upgrades more painless, especially for apps. The one problem was Pop3 emails. Think of the coding issues: how much space do you allot for Pop3? How can you make sure all sorts attachments, some custom-coded are uploaded? Definitely not cookie cutter. My theory is that their current solution is to limit the space occupied by emails. There are several possible ways to do this. Just like on your IMAP work server they could just limit the size of the email. That probably would have brought even more howls. So my theory is the new IOS has a cap on mailbox capacity. That would explain why the so-called Pop3 problem is erratic. In essence Pop mailboxes have become too much for IOS.
The limitations of IOS and the Pro put Apple at an interesting crossroads. If the Pro really is going to compete with Microsoft and Google in this market IOS does not cut it, as numerous reviews have pointed out. It needs a real OS. I am wondering if with the new chipset whether Apple will eventually do that. THAT would be a game breaker if they put it out some time next year. The very real problem is programming it to run with legacy IOS apps. They have faced the equivalent of this when chip architecture went from 32 to 64 bits. Apple did the right thing. Windows screwed up which is why a lot of folks switched. Now Apple has a chance to change the game again. I would say a new OS for tablets and phones could bring a market share uptick of ten points or more! especially if they could have robust links to Apple TV. Good luck Apple! A lot of us are pulling for you.