Re: Lollipop killed stock "email" app!
No offense, but I have no idea from all that you wrote why you use POP3, especially for Gmail accounts, or what you are doing with POP3 that IMAP can't do much better. From what you say, IMHO you're doing things the hard way just because it's what you're familiar with.
If you want to keep email on the phone so you're not dependent on an internet connection to read mail, simplify set the email client to sync lotsa mail. I have several accounts set to sync 1000 emails.
You said something about you're used to how POP3 allows multiple devices to view and manipulate email. But isn't that the biggest weakness of POP3, that email that has been read, replied or otherwise dealt with is not synced to other devices? For example, you had to set POP3 Gmail to forward a copy of sent mail to the inbox for other device's. With IMAP it's not necessary, sent mail syncs everywhere. It just works.
IMAP can do much more. With POP3, almost all operations must be done on the device or computer. Filtering mail sorting mail into folders, creating folders and much more has to be setup and done locally, one device or computer at a time. With IMAP just setup filters, folders, etc one time on the server side and you're done. Everything - mail filtering, sorting into folders, etc, whatever you setup - automatically syncs to all devices and computers. And of course mail handled locally on your phone automatically syncs to everything else.
And there's those handy push notifications in IMAP.
If you don't like the conversation view in the Gmail app, (I don't) just use another client. There's plenty to choose from - Type Mail, Maildroid, Cloud Magic, K-9, My Mail etc. Those are just a few. My favorite is AquaMail.
Try a few, and give IMAP a chance.
Android since v1.0. Linux user since 2001.