For Merrill specifically, because she doesn't appear to have been played much, it's okay for you to essentially reboot the character and have her arriving at Skyhold for the first time. Just make sure to make that clear in an intro post or something similar.

Generally (for the future FAQ reference of others), this is something we'll handle on a case by case basis. As the game proceeds and characters build up substantial records in the game it may not always be possible to offer a completely clean slate. But we've asked existing players to be flexible with re-appers and we don't intend to automatically lock them in to every detail of previous RP. We're always willing to discuss options and open to working with new applicants and existing players to help them find a compromise.