Hi Sarah and Stormy,
You ask great questions and have been given excellent advice already.

stormykitten said
Stormy is having her stitches out tomorrow. I am going to ask the vet about the following things, but would love the group's feedback. I am very worried about her recovery and want to make sure she has everything she needs to feel better.

Question 1: When Stormy walks, she tries to lift the missing leg up and this causes her to unbalance and sometimes fall over. She also meows or cries when this happens. 

Yay for the itchy stitches to go!

I would video this so you can show your vet. First off, I'm not a vet & Purrkins is a front leg amp.! I agree with Sally faceplants or tipping over is common as they learn their new body, but with vocalization, it sounds like she is having some nerve pain.

Question 2: The stitches look good, dry, and I can't see any problems with them. However, the site twitches sometimes. What could the twitching be? 

Twitching alone without reactivity is normal and can be ongoing.
No reactivity means no vocalization, attacking the missing limb, biting, licking, the missing limb, backward walking in rear leggers. If there is any of those, it is nerve pain or phantom limb pain; the gabapentin would need to be adjusted dosage/frequency to help Stormy.

Question 3: Stormy has been nerve pain medicine (gabapentin) and pain medicine (bupernorpherin) "as needed" but I worry she's still in pain and that the above questions are related to nerve pain. How can I TELL if she's in pain and still needs meds?

How was the buprenorphine prescribed? How often have you been giving it?
I don't care for as-needed instructions, just saying we had major surgery it is needed. We always want to stay ahead of the pain and not try chasing it.

What is the Gaba regimen? Depending on dosage and timing, I'm sure they can tweak the Gaba - I would ask today.

Purrkins was every 8 hours.
Hit any pain signals link for even more information on what to watch for. From your questions and concerns, you do know what to watch for. I know cats are masters at hiding pain, but your observations tell me you are on this.

Question 4: She has been very tired and sleeping extra. She still gets up to eat and walk around about once or twice a day, but is otherwise content to sleep in bed all day. Is this normal for day 12 after surgery?

this may be the gabapentin; it does cause some sedation at first. Sleeping is healing, but please do bring up your concerns to the vet.
We need to remember you are watching Stormy more in recovery, possibly right? Do you know how much she was sleeping before?

Did Stormy have presurgery bloodwork, and all was ok?

Good to hear she is up eating and moving, going to litter, ok?

Try to think about it in human terms. How would we feel 12 days after an amputation? It puts things in perspective more.

Purrkins was on the same Gaba for 14 days and buprenorphine for 12 days. Then he went to acupuncture to help get off his pain meds. Purrkins had little bouts of energy and rested both. On day 14, Purrkins got his stitches out and was free to be a cat again.
Purrkins was 6 when he had his amp.

Let us know what the vet says.

Scratches to your beautiful Stormy, please.

Hugs, Holly & Purrkins💝💝💝