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.