This comic follows on from the Previous comic which will almost certainly provide context.
The spell Konsi’s talking about is “Fortune’s Favour” - it’s level 2, consumes a 100gp pearl to cast, and lets you re-roll a single D20 within the next hour. It’s in Explorer’s Guide to Wildmount.
It’s been a little while due to drawtober, and my website is (mostly) functional again, so if you want to read all the Konsi comics from the beginning, you can do so on my website at this link. Please be advised that these posts are presented in reverse chronological order, so start at the end and work backwards.
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).