But after changing that setting, anywhere I need to compose an email would use the default account I nominate, no matter which.You would think but no. It's in the mail app after you press "edit".
Move iCloud to the bottom. The iCloud at the bottom, under accounts. Not the folder structure.
I believe my setting is the correct one, and I don't need to mess up with display order, as in iOS 10, you cannot use your method anymore.
Update: sorry for wrong info. You can still reorder account display order in iOS 10, just a little bit tricky as you need to collapse all accounts before re ordering them.