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).
Fortune's favour would be pretty useless in this kind of situation, as it takes a minute to cast, you can't subtly re-up it during the game. You could pre-cast it and get one favourable outcome, but that's not enough to win a tournament with, and the 100gp cost would mean you're probably just wasting money. On the plus side, it's not concentration, so you could put the buff up on top of enhance ability or borrowed knowledge.
As for the house rule, what's happening here is tomfoolery and japes.
As for Konsi's access to this spell in specific, most of the spellcasting NPCs in our setting were created and had their spell lists defined early in the campaign. EGtW came out after they were already long established - the party actually knows Addis' spell list (it's a static list, consisting of a subset of the full cleric list). Konsi's sudden acquisition of a new luck-based spell, as an outcome/reward of her completing a major goal of her goddess, does make some kind of sense as a "special" situation.
As to whether or not Konsi is a "chosen" it's a little ambiguous. Tymora already has a defined chosen in canon (Curran Corvalin.) Gods normally only have one chosen at a time.
Tymora hasn't explicitly made it clear to Konsi that she's chosen, just that she has a mission to fulfill, favour and support. Likely, Konsi's on the path to becoming Tymora's chosen, but she's not there yet, she's just an important priestess that's going places.