Email from iPhone

1. Not entirely sure what Apple complicates.
2. The iPhone will automatically set the "From" address to either the current accounts email address when you reply or the default email address that is set in Settings->Mail if you start an email in the All Mailboxes listing or outside the Mail App. You are of course free to change the email address from where its sending at any time, by tapping in the CC/BCC line and then the "From" line to choose a different address.

There is no complication whatsoever.

I'm just a user like you, not a tech genius. That said, if you set your default account in Settings>Mail>Default Account (last option at the bottom), you shouldn't have that problem.

Regards

Giulio