That really doesn't work either, Harry still burned Lara's lips in White Night five years after his split with Susan. True, she was still alive, but he had had no contact with her at all in all those years. To my mind, the love you feel lingers more after one loses a loved one to death than a split up, because you are still loving the memory and there is no distraction from it.
Personally, I agree with you about the love lingering more after a loved one dies, especially if one is summoning the shade and playing boardgames with it every night. But narrative necessity triumphs over all.
They might not have had contact, but Harry still felt love for Susan after 5 years and hadn't been with someone else. Susan still felt love for Harry after 5 years and hadn't been with anyone else. Ergo, lack of contact alone doesn't break the protection.
The problem Jim created was that Harry burned Lara while Murphy was alive, but not 4 months later. He needed Lara to rock Harry's world with a superkiss on Halloween which should have burned her lips off if the protection held. His excuse of "I just fed" from the first date won't hold if she is indeed feeding on him.
Second alternative, Jim could have said it was Odin's scooping her up that broke the connection.