Android requires at least one-working Gmail that is apparently tied to the default email. Now I would like to have that email under K9 but I cannot because Android instructs me to hard-reset my device if I remove the Gmail from the default Gmail. Now the problem is that I get two notification from Default client and from K9. So how can I have all emails under K9?
You don't need to remove default Google Account or factory reset to ditch double notifications. Use any one of following methods which suits you best:
Accounts & Sync, choose the default Google Account & turn off Gmail Sync. After that, you'll receive notifications from K-9 Mail, ONLY.
In some Android versions, if you turn off all sync options listed under a Google Account, a Sign Out button is appeared. All you need is to tap it. Go ahead to try it.
If you are rooted, delete
/data/system/accounts.dbfile & restart the system (On Jelly Bean, its
/data/system/users/0/accounts.dband the location can vary with Android versions). Next time, you'll be asked to add Google Account again.