Fortunes Favor doesn't seem like the optimal spell to use in a situation like this (even if you could). Unless the games include no social component, enhance ability would be better.
About the house rule: I get why you'd use the rule for simplicity, but especially if your not playing a cleric, but instead a wizard or an artificer, it seems weird to make the PC a chosen one by having those spells.
I usually give NPCs a set selection of spells they use. But this set is only their prepared spell-list, it varies depending on the NPC and most preparation-casters will have access to the other spells on their list, should the need arise (as it most likely won't, so there is no extra work on my part).