Could repeatedly falling asleep at the breast have triggered "baby bottle syndrome"?

Yes. Nursing caries. Can occur from repeated exposure of the lingual, or tongue, side of the. The teeth to the sugar in breast milk, especially if the mother & baby have streptococcus mutans bacteria in their mouths. When mom & baby fall asleep, the nipple retracts, allowing breast milk to bathe the baby's emerging teeth repeatedly. At 6 months, night feeds become a habit. See healthy children. Org for safe sleep.
Unlikely. Unlike bottles, most mothers do disengage once the baby is asleep. Of course, a degree of caries is possible even with breast feeding.