I'm going to respond to your questions separately.
You say she opened SNAD and you say you asked her what was SNAD about the bag and that she didn't respond. However the dispute would show her claim and what she perceives is wrong.
1.
So, on delivery date she opens snad before the tracking # showed delivery.
That's not an issue. Generally tracking doesn't update until the carrier completes his daily deliveries and returns to the postal facility where they download all the scans for the day. Once that happens, tracking WILL show the date and time of delivery but again, it might hours after delivery occurred.
2.
They said it was buyers remorse and I shouldn't of accepted the return or paid for return shipping.
I'm not sure that ebay gave you good information. They're notorious for telling callers what they want to hear and often, it's not accurate.
She claimed SNAD and you accepted the return so return shipping is on you (seller) in SNAD cases.
3.
They refunded the sellers fees and said I should dispute when I get the bag back and I do not have to refund shipping or the return shipping.
That was nice of them to credit the FVF but again, for SNAD, you're on the hook for shipping in both directions UNLESS you fight the claim when it's made.
4.
She's only had 18 purchases, but her feedback was good.
All buyers have good (100%) feedback because for the last 8 years, that's all sellers can leave for them!
What about the feedback she left for others?
5.
Any suggestions on how to weed out buyers like this before I ship?
You can't block or prevent a PITA buyer who does a BIN. However you can set your buyer preference restrictions at the tightest level to prevent them from buying. But that only works if they've met the criteria.
You can add her to your BBL and help other sellers too by posting her ID here. It's not just for non-payers:
*E-Bay Non Paying Bidder List*
6.
I doubt I'll get the bag back.
If she doesn't ship by the date required in the dispute and if you don't get the bag back, you'll keep the money. Your only loss will be the return shipping label you'd paid for but that's a small price to pay.