We've had dental work done on both kids with general anesthesia. The boy had tubes put in his ears with just gas, not IV drugs. He was a screaming banshee when he came out from the gas. It was still on his breath 30 minutes later and he was still screaming. They said that's how most kids come out of gas anesthesia.
With the IV meds they go to sleep like anyone having general surgery. They sit there watching TV until it's time to go back, they go in and get a little gas to knock them out (not totally sure about how much and your dentist might just do the IV with them awake). Then they go to sleep and wake up totally in an hour or less with new teeth. The dentist can do all the work in a few minutes with them asleep. It's so much easier than any other way and we will never have any dental work done on the kids without them being asleep with general anesthesia.
Go for it, it's so stinkin easy on them, hard on us but easy on them. They wake up ready to go eat and play for hours since they're so rested. It's very easy on them.
To address the decay, she probably just has weaker enamel. All food is a problem with teeth to a point. Not just sugar and other foods. All foods can cause decay if the enamel is weaker.
I have weak enamel, my daughter got weak enamel from her dad and from me. All her kids have had various amounts of dental work done from nothing more than a good cleaning up to having a bunch of silver teeth. She treated all of them the same, they all ate the same foods, they even went to live with different people after a while and they still had varying degrees of dental issues within those homes too.
So don't beat yourself up, she would have gotten the cavities most likely anyway.