First, I understand the 90/180 days rule

Suppose you have a multiple entry, 1 year Schengen tourist visa, with a duration of 90 days.
1) Theoretically, can you stay in Schengen more than 90 days in a year (observing the 90/180 rule, you can stay up to 180 days in a year), or between your visa dates (1 year period) your max stay is 90 days, no matter what? (until you get another tourist visa perhaps)
2) If the answer is 90/360, practically speaking, is this enforced as much as the 90/180 rule?
Related: I know enforcement depends on many variables, but let's assume all those are the same. Can you rank the enforcement/seriousness of the cases below:
A) Departing from Schengen AFTER the visa expiration date, regardless of duration spent in Schengen
B) Obeying the 90/180 rule AND the visa expiration, but staying in Schengen longer than the allotted days (90 in our example) in the visa validity period (1 year in our example)
C) Breaking the 90/180 rule, which probably automatically includes case B as well.
3) "Bonus question" about dual-citizens: In practice, does 90/180 apply to the individual or the passport?
I assume if both passports are of third country nationals (meaning requiring visa), then since they are giving biometric info at the entry, they can be cross referenced easily. But if one or both passports don't require visas, then no biometrics are collected, and even though the passports themselves contain biometric info, they probably aren't cross-referenced (assuming due to anectodal evidence from friends. Asking for more anectods or accurate policy info).
Thank you