I'm dealing with this now, too, and for the SECOND time. :( My older son had to have five caps across the upper front teeth and two crowns on molars when he was 3 1/2. Now my little guy, at 23 months, has a ton of decay. I took him in just before he turned one for a broken tooth and he had no decay at that point. For the past few weeks I've been trying to get a good look at his teeth and between bad lighting and him being squirmy and not wanting to show me, I just couldn't get a decent look... until I thought of looking while he was strapped into his car seat on a bright sunny day. And, omg! :( I knew for sure there was an issue on one of the upper fronts and suspected more, but when the dentist had him under the light and looking through his mouth with that little mirror, it was nearly every tooth except for his front bottom.
In Augie's case (and probably Milo too, I'm guessing) there is way more than night nursing in play. The dentist said they were totally malformed, and some of them looked very oddly shaped to me. And that's probably why he broke the one he did. But both of my kids are very busy during the day and hardly eat food. As toddler's, though, they were (are) both latched on practically all night. I really do think nursing contributed -- though I doubt it's any worse than allowing a baby to be half sucking on a bottle all night long. My kids are at risk for tooth decay bc of the quality of their teeth and then letting them nurse all night is a risky behavior. Part of the reason I feel so strongly that it contributed is that we had Milo's teeth fixed about two weeks after he stopped nursing and he's never had another cavity.
Otoh, I do not regret extended nursing, or even nursing all night long. I feel that all of us, and especially my boys, have gotten so much out of that and it is so good for them and completely worth the devastation of their poor little baby teeth. In Milo's case, fixing his teeth was expensive, but no skin off his back. He thinks of it as the day that he was allowed to watch dvd's all day long and doesn't understand why we make such a big deal of taking care of his teeth since fix them was so easy. General anesthesia is a little scary, but doable, imo. For Augie, we are going to bring him in to the dentist every three months until we get them fixed. If the dentist sees signs of possible infection or if there's any pain, we'll get them fixed right away. I'm hoping, of course, that the teeth will hold out until we are ready to wean. If we're close to weaning anyway, I'll probably just go ahead and wean and not nurse at all after the surgery, since it worked out so well with Milo. If it's anytime soon, ugh, I guess we'll have to cross that bridge when we get there. Right now nursing is super important to him and I just can't imagine stopping. :(
As for the theory about mama sharing bacteria... I was really worried about that bc my teeth are horrible (they weren't when I was their age, though). But if that were what was going on with us, I feel pretty sure that Milo would still be having problems. I'm sure I was sharing a significant amount of bacteria with him when he was 3 and 4 and probably still am and he's fine.
I'm happy you decided to put your LO one out for her work, OP. I have had so much dental work done myself that the idea of doing that to a child, ugh. I'm so happy Milo was out and it was really easy for us. Well, my husband was actually really freaked out and upset by it, but it was very easy for Milo and mostly easy for me. I hope it all goes well for you guys!