an incident can be positive and even expected, it is more like "an action" - no judgment. An accident - always unexpected, and has a negative connotation (even can be applied to some positive outcomes )

"Incidence" is similar to occurrence - it is about the frequency with which something happens - the incidence of serious illness with covid is much lower in in those who have received vaccinations. "Accidence" is a grammatical term linked to changes in the forms of words: tense, person, case, number, etc.