I don't recall age restrictions on games back in the day, not even for risque pc games like Leisure Suit Larry. I wasn't really looking to buy such games, though, so maybe the clerks woulda told me to get lost if I tried to buy one.
As for Splatterhouse's suitability as a pack in, I don't think it would've helped. It's a better game than Keith in every way and might've helped with some prospective buyers, but the 'mature' content probably would've scared off more than it lured in.