As far as this one (setting manually for iMAP), yahoo did not allow the sign in:
Your account is currently not enabled to sign in from apps that do not meet modern security standards (ex. Older versions of mail and calendar apps such as Outlook). As a result, we prevented a sign in to your Yahoo account.
So then I went to Yahoo security settings
Update your Yahoo account information and settings | Yahoo Help - …
and set
Turn on Allow apps that use less secure sign in.
and now it is working. Anyway, to experiment I am leaving one Yahoo mail set like this, and one regular auto settings BUT with security settings set to allow less secure sign in. Will compare - so far, both are receiving all emails so it seems like the "less secure" setting is what matters, not so much the manual iMAP configuration.
Until this sorts out I would suggest regularly checking your email via web based yahoo tho.
Also, email received via the yahoo app on the iPhone arrives just fine and instantly, and you may add multiple yahoo emails there....