[i]It seems you do a sharp right turn in the iddle of your manifesto. Words do change their meaning over the years. But cuckold has held its meaning for 600 years at least. Implicit in the explanation is that the husband may or may not be aware of his wife's cheating behaviour.
It is only my idiocyncratic view but I cannot see the word lending itself to describing MMF sexual encounters where their is a married coupe with another male and all three are unvloved. This has more to do with sexual offense and domination of the sissy husband (wish there were some other words to describe him) and is not purely voyeuristic.
An open marriage where a loving couple may have extra marital relations that each other knows about isn't strictly cuckoldry either. But because a phenomenon has no precise name shouldn't preclude it from being valid or even very enjoyable!!
I thing dogging (in the UK sense of the word) is probably driven by a penchant for voyeurism. The Italians have for centuries institionalized the implicit promiscuity of their wives by acknowledging the cavalierie servente (the accretided lover) but have been quite ruthless in their dealings with unfaithful wives who have unaccredited studs as it were.
We can argue about the exact definition and application of the selected words but we should not question the consensual activities of people.