Hi....if it makes you feel any better, I've had at least 3 root canals (lucky me, grew up on nonflourdated water) while breastfeeding my 2 kids. I had standard dental anesthesia, took Motrin for pain, and amoxicillin. We had no problems, and my endodontist (root canal specialist) was great. I'm sorry you had to go through so much.
I am a NP myself and the lack of knowledge doctors and NPs/dentists/nurses have about meds and breastfeeding absolutely floors me. It's one of my professional pet peeves.
Oh...take it from one who knows, root canals aren't so bad. Think of it as a great big filling. You'll do fine! Good luck!!!