Thanks for writing up this primer on Fenton Oath! Like many aspects of Vintage, there's a 'wild west' feel to deck/archetype theory and primers like these really help establish a baseline for each archetype, similar to the DTB and established deck section for Legacy at the mtgthesource.
I love this deck and exclusively played this deck for a full year in paper tournaments (NE weekly tournaments, NYSE, Champs etc.). I know nothing about online so here are my 2 cents based on paper games only:
LSV actually played a similar list at vintage champs last year, he was 7-1 before having to drop to catch a flight. So ignore Pikula, oath is real deck and can be a great choice for some metagames.
This is not the best JTMS deck for sure, but like you alluded to, the 'brainstorm' effect is absolutely insane and necessary for this deck. I've been extremely happy casting JTMS while giving my opp a token, because I still get 3 brainstorms out of it before it dies, AND Jace can always tick up. JTMS is also by far the best card in the mirror, so I'd be very reluctant to not play at least 1, usually 2, and have been happy with it.
On a note re: the mirror - Yes traditionally you sideboard out 3 oath, and 2 show and tell. You win the following way: After establishing orchard dominance, tutor for your single oath, hardcasting griselbrand eventually or stick a JTMS. If you play vault key, that is another way to win obviously.
My main issue with the deck has to do with consistency. There are an upward of 6 cards (3 grisel, vault/key/memory's journey) that do almost nothing on their own in the deck, and having any of them them in your opening hand (50% of the time by my estimate) feels bad. You can get lucky and even spike a tournament or 2 with great draws, but eventually the variance will catch up to you.
The other issue is containment priest. While abrupt decay is a great answer, mentor just does not give you enough time to find the decay due to how quick of a clock it is.
While winning game 1 is easy, 2 and 3 becomes nightmares IF your opp is well prepared. When ppl are unprepared against oath they get crushed.
Completely anecdotal, but fun anyways: Last weekend, I played against Greg Fenton and he is on the play with the following draw: lotus, recall, oath x2, orchard, land, flusterstorm. I beat that god draw with a turn 1 aegis of gods plus double force of will hand. If the game dragged on longer, he might have found an abrupt decay, but because my deck has more velocity, I was able to find a containment priest before that happened and seal the game completely while he just sat there playing draw go for awhile.