I hope I can be of some help, I am unaware of anyone else on here who battled a NST, I am hoping someone else may chime in too. NSTs are a type of mass cell tumor but they present differently. My Jake had the exact same thing as your Zoe, a malignant peripheral nerve sheath tumor and the "fingers" of it had reached really far down his leg and thru his shoulder. He actually had it for 4 years (it was misdiagnosed by our vet as a fatty tumor that she didn't feel was posing an issue). The good news is that they are locally invasive, and the key is removing it with good margins. I'm guessing by your post that your vet didn't tell you whether they got good margins? When they removed Jakes, the surgeon said the chest wall is a good barrier and so he removed everything from the chest wall out. You can feel Zoe's amp, does it feel like its pretty concave? This is what Jake's looked like a couple days after surgery but they had to removed ALOT of his shoulder b/c of how much it had invaded. You can see the concave part, that's his chest wall. 

I know you didn't get the results but for NST's usually anything a grade 2 or under has a great possibility of not returning. Jake's was a grade 4 but he was NST cancer free for 9 months. He developed a 2nd cancer, a hemangiosarcoma, that the oncologist said was completely unrelated to the NST and that it is really rare to have these two forms together. He kicked the NST's butt. 

When did Zoe stop Gabapentin? We had Jake on it all 9 months because of phantom limb pain. The way the vet described it was that when they cut the nerves, they still function. They try to fire off but can't go anywhere. That sounds like what Zoe might be experiencing, and it can last indefinitely to my understanding. I would give your vet a call, they can do a feel around the amputation to see if the NST is starting to grow back and see about putting her on Gabapentin again. Especially with this type of tumor since it affects the nerve's specifically. If it does come back (which they are known to do without clean margins), just try to remember that they don't tend to metastasize. 

Please feel free to PM me anytime. I don't always check the threads but it'll send me an email if I have a message in my inbox.