Of course, Valanice wanted to get Rosella married off, and Rosella wasn't necessarily in a hurry to get married. This was right on the heels of King's Quest VI: Heir Today, Gone Tomorrow, where Alexander had just married Cassima, and taken the throne to the Land of the Green Isles. It meant Rosella was still (or went back to being) the primary heir to Daventry's throne. To Rosella, it was "What's the hurry? Alex and Cassie can handle producing grandkids." To Valanice, seeing her son married off was reminding her that it was time for the daughter to settle down, so that Daventry would have a successor.
Edgar has been Brainwashed and Crazy, transformed into a Troll King impostor, is being watched like a hawk by Malicia, and knows what she's doing is wrong, but is too confused to know what to do, how to stop her, or even understand what's really going on. When was the last time he was in that situation? When he was being "raised" by Lolotte. And who saved his butt that time? Rosella. And what granted him the opportunity? When Lolotte tried to force her to marry him. By kidnapping her, and trying to invoke And Now You Must Marry Me, he was actually trying to call for help from someone he trusted, but was too morally compromised and confused to do it properly. This also makes the good ending an extra layer of heartwarming; he wants to give Rosella than chance to get to know him as a person, and decide for herself if she loves him, without all the crazy world-destroying business complicating matters.
Fairies are implied to live much longer than humans in the King's Quest universe. This means that Edgar signed up for a Mayfly-December Romance with Rosella, and who knows what it would mean in the Telltale Games sequel for Gart.
The Fan Remake of the second game already brought this up, but with Alexander in the Land of the Green Isles, and Rosella ruling Etheria with Edgar, there really isn't an heir to Daventry. Well, unless you accept the idea brought up in the Air Gem test where Graham makes Connor (protagonist of the eighth game) the First Knight (and heir) the way Edward the Benevolent did for him.