Have you considered setting up forwarding on your gmail account? That's what I do. So people still email me at my gmail account, but it comes into my iCloud account. When I reply it's obviously from my iCloud account. Then you can just remove your gmail account from Mail and you'll never accidentally use it.
This might not be satisfactory if you sometimes want to email from gmail on your phone. but if those cases are rare enough you could get the gmail app, turn off all notifications etc., and just use it as a client for the occasional email, as i do.
edit: realise this is not exactly what you want, but could be of use to your or anyone else in similar situations.