Please do not return your pup. I truly fear for her life if she is returned.
LP is workable. I was in the same boat as you. I have 2 dogs both with LP. And YES it is hereditary. Our dogs were born with the genetic make up for LP. Sure, their patellas aren't luxating at birth (although some cases are), but sometime shortly down the road though, those knee caps are going to start to slip and that's b/c it's in there make up.
Roxy had it worse than TJ. Roxy has very short legs and she NEVER was able to jump up or down from furniture or go up or down stairs. I lifted and carried her at all times. She was first diagnosed at about 1 year old. I had noticed a clicking noise for quite some time, but I didn't think anything of it really. At some point I brought her to the Vet for dental and I mentioned the clicking and he checked her legs and said LP - Grade 2 and 3 in both legs.
She has had surgery to repair and she is a brand new girl.
TJ has LP too, but not that bad. We are supplementing both of them and I have TJ checked every 4 months. If the time comes that her too needs surgery, than we will go that route.
Best of Luck!
P.S. Do a search here on yorkietalk for Luxating Patella
There are soooooooooooo many threads, you'll find lots of great info.