Plus, at some point Thomas leaves the island. Certainly he will have been changed by the experience. He might even become a KotC. Imagine what that would look like to other members of the White Court. To the monsters it would look like Dresden did something to change Thomas, to alter his normal; to them, demonic side and make him into something they can't comprehend. And it won't matter a bit that Harry didn't actually do anything to Thomas, other than save his life. It would sure look like Harry did something. None of this would be good for Lara. To other members of the White Court it would look like a mortal; a powerful wizard true, but still a mortal human, is the more powerful individual in a relationship with a White Court vampire. This is virtually unthinkable for any member of the White Court and would make Lara look weak.
This doesn't mean Lara couldn't recover from a situation of this type. Maybe this would be when she demonstrates that she learned her father's kiss of death technique. However, it would certainly complicate the situation.
I know there are other readers who think that when Thomas returns from the island he may become the next King of the White Court. I'm not sure how this would happen. The only thing I'm certain of is that when Thomas returns there will be unexpected and serious repercussions.
And here's a completely different issue which could complicate Harry and Lara's relationship. In Peace Talks, Lara confirmed that her father is starving to death. We don't know when it will happen but when it does the masquerade that Lara has been playing; that her father is still in charge and she is acting as his deputy, will be over. Someone in the White Court is bound to make a move against her. It might involve making a move against Harry first.
So lots of fun possibilities to make things complicated, make that more complicated, than they already are.