If the team complained about their code base for _years_, I'd say the quality of the code base they started out with wasn't the only problem. Especially considering that you had the resources to write an email app from scratch.
While it's true that parts of K-9 Mail's code base are gruesome (even today), I can't remember any significant contributions from teams that have built products based on K-9 Mail.
Agreed. The entire Android side of the product was not managed well from the start. The product started as iOS only, because top management had iOS devices. Suddenly they had the shocking realization that Android is very strong in Asia and Europe, and tasked a different team with creating a quick Android POC. That team chose the K9 codebase. The POC team then handed whatever they did to a new dedicated Android team which did all the complaining. They were a small team, but still. My team wasn’t really larger, and we did build a mail, contacts and calendar client. I guess management just couldn’t agree on the commitment.
Switching between the F-Droid version and the official APK (Google Play version) is not possible. You'll have to uninstall the old version first (and that will remove all settings). However, K-9 Mail supports settings import/export.
