Erembour isn't his fetich. She's his Seventh Soul and given her attitude, she likely embodies his capacity to love things. Taking that from him would result in the most fiendish being in existence getting
even worse.
That said, the example you brought up certainly shows its possible. Granalkin loved/respected Luna enough that it conflicted with his loyalty to Mardukth, so the Sidereals were able to use that "demon to god" charm on him, most likely.
Doing such a thing without the Yozis' permission will certainly result in every other Infernal and a host of demons being ordered to kill us and the traitor demon. The Ebon Dragon might love us for chasing such a doomed love, but oops! We took that part of him away.
So while romantic, it's also really stupid.
Also, goddamn man. She just kissed Lelouch on the nose. Cart before the horse, much
Erembour, like many TCDs, has multiple partners. She has relations with those who keep hidden loves and is summoned to teach people how to do sex really good. This isn't a girl we can marry or "claim" in any permanent way. She's too long-lived and powerful for a thing like monogamy to have hold on her. Probably the most we could do is get her to see us as a friend-with-benefits or something.
One final edit: On the note of her being too powerful, try not to think of her as a girl, lady or woman. She's an incarnate force, an aspect of a much greater being consolidated into its own persona. So while you're planning to woo her, please keep in mind that respect is an integral part of any conversation.
All that said, I am totally for getting her in the sack.