A simple answer to your first question is no, it's not unprecedented. The first Super Mario Galaxy had a more limited version, and of course comments and answers have mentioned other games as well.
The last question's easy to answer: there's certainly a better name than "wife mode". Sexism is rampant in gaming, and while "wife mode" might be accurate in individual cases (as in your own: after all, it was your wife who used the name), as a general term, as you might suspect, it implies two assumptions that could be taught in Sexism 101:
- The person playing the secondary character is the wife (or girlfriend or whatever), because the primary gamer in the house is the man.
- The person playing the secondary character needs extra assistance (invulnerability, doesn't have to follow the plot), because that person is not an equally-skilled gamer, which is because that person is the wife.
Some women are not avid gamers, and some women are not skilled gamers ... but it has nothing to do with their gender, despite the existence of places in the gaming community where that idea is accepted as fact. So there's no reason to use a name for that mode that suggests it applies to only wives or only women.
There are plenty of gender-neutral terms we could use that would still be accurate. For example, the original Super Mario Galaxy calls it Co-Star Mode. I think that's actually a pretty good name: co-stars are not necessarily as important as stars in TV shows and movies, but they can contribute a lot on their own, and of course in the context of SMG, there's the Star pun (intended, I'm sure).
Assistant Mode, Helper Mode, those would work as well, and I'm sure there are more like that ... and none of them make assumptions about the person who'd be playing them.