Cassie Claire wrote a fanfiction called Mortal Instruments. Its been deleted for obvious reasons (the same obvious reasons her pen name is now clare instead of claire) but it did exist and you can still read it if you know where to find it, no I wont tell anyone where.
You are corrent though that her Draco trilogy, of which Draco Dormiens was only the first installment, has nothing to do with the MI series. (though she did take a section out of one of her fanfics and readapt it for MI, so maybe just a little bit) Its just a lot of her fanbase says Draco and Jace are the same person, its not like all of them say it in a bad way. But as I said in that other post though, I wouldn’t know if they were or not. I can only go by what majority of people say. If you disagree, thats cool. But I can’t say you’re right anymore than I can say they are xD