Yes, It is very complicated. The simple rule to remember is there is no simple rule.
A couple of answers, however, to your additional questions:
A '77 model year car could easily display '76 plates in certain circumstances. In that era, most states issued license plates for a calendar year. Thus, '76 plates would be issued during 1976 with expiration date of Dec 31, 1976. During that era, most American cars were introduced in September before the model year designation. So, 1977 Firebirds went on sale in September 1976, for example. A newly purchased 1977 car would have been issued a license plate with an expiration date of Dec 31, 1976, thus it would have displayed a '76 sticker.
Sidebar: Most states no longer use the calendar year, but issue plates with an expiration date a year after registration. This was done to spread all the state's renewals over all 12 months rather than having all renewals hit on Dec 31.
And, whether one plate state or two plate state, the validation tabs or stickers are placed on the rear plates, not the front plate.