How to assign 1 of 4 email accounts as default?

I have 1 primary email account. I just added 3 more email accounts to the iOS native mail client. I have 2 problems with this arrangement.

1. When mail is called by a third party app, my (what I consider) primary email is most often not used. I then need to edit the 'from'. I'm not finding where/how I can stipulate 1 of 4 to be the default?
2. Of the 4 accounts, I'd prefer that 2 of them display in the inbox. As it is now, the only way I know if there's mail in that second (the other 2 do not need real time acknowledgement) is if I click on the current mailbox name top left of Mail's inbox and view all 4 accounts.

Mark