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 there 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).
Ah yes. Good ol’ Volo. Nice to see him from time to time. I should use his books more often.