At the end of the dominance when you discard melisandre's favour do I keep control of the character or give it back

The control of the card reverts back to the owner if there are no effects currently modifying control (like, say, an enslaved or Ygritte etc). So when Favor drops off, in the majority of cases the card goes straight back to its owner's control.

What about and event card like seductive promise does the change of control only last for the turn?

No. If a triggered effect (like Seductive Promise) does't give a set duration, it is permanent. If an attachment's constant effect doesn't give a set duration, it is only active while the attachment is on the character.