So I'm changing realms (my best mate is coming over but doesn't want to play Hib).
Since I want to keep my char names, I'm going to delete all my hib chars (lucky enough there's not that many), but my question now is: once I deleted them, do I have to restart the game or can I just click the "choose realm" button within the client?

Because I don't want to delete my chars and then face a 3+ hours queue. Does the 10-min grace period apply to this and does it work as intended?

You do have to exit the game completely to change realms after deleting.
I believe that you will have a grace timer from queue though.