Faithless Looting is perfectly fine as a 4-of, because unlike Once Upon a Time, multiples (2) in the opening hand are castable without taking away from going off on turn 2. Drawing 3+ in an opener is obviously not what we want, but it's still serviceable. Once Upon a Time is a card you love to see in the opener, but you really don't want to see multiples. Having a 2nd copy isn't necessarily horrible, and they are usually fine to topdeck into (assuming we have the mana), but it's definitely better to run 4 Looting and 3 OUaT than it is the other way around.
I believe Lotleth Troll is necessary, but not every one is on the same page with me. The thing is though, you have to have an initial discard outlet creature before the rest of the deck does anything. Sometimes, you don't have a Cavern of Souls and the first one gets countered. Sometimes, the first one gets killed after a mediocre opener and we need another one to go off again in the midgame. I'd much rather have an initial discard outlet in the opener, and use OUaT to dig for a Hollow One or Vengevine. Keeping a hand with a OUaT without a discard outlet creature is much riskier than needing any other piece.
Lotleth Troll gives us a non-graveyard dependent large trample regenerate creature that is a powerhouse in a lot of matchups. More resilience to Chalice @ 1 is also really important. Maybe they could be trimmed to 1 copy, but I wouldn't cut them completely. I also don't think it's the sort of card you should run copies of in the sideboard. There are better cards that address matchups more significantly, and I already don't have the room for all of the cards I'd like to have (especially the 1-of Pithing Needle that I had to cut).
Stain the Mind is just way too good for me to not want to run in the maindeck. Most matchups have 1-2 cards that we care about, that Stain the Mind deals with. Even just being used as a discard spell against redundant fair matchups in tandem with Cabal Therapy, it's still a solid card. It's also castable in multiples in later turns without tapping impactful attackers by spending more actual mana to cast it. Obviously the first cast tapping down summoning sick attackers to cast it for 0-1 mana is busted, but it's not out of the realm to tap something like a Putrid Imp, Basking Rootwalla, and Bloodghast along with 2 lands to cast a 2nd copy on the following turn without reducing the amount of pressure we can apply too significantly.
I'm pretty happy with 59 out 60 cards in my maindeck, with a single flex slot that I'm not completely sure about, but I'm leaning extremely heavily towards Stain the Mind right now because of how powerful the effect is and the fact that I'd love to be able to cast one in almost every game.
Aside from wanting to fit the 1-of Pithing Needle back into the sideboard, I'm also 100% happy with those choices as well.
You're certainly welcome to tweak and playtest your list until you find a configuration you're comfortable with, and I've certainly tweaked mine around quite a bit myself, but I'm extremely satisfied with where I'm at right now.
EDIT: Maybe Tomik is the cut for Pithing Needle in my sideboard? Needle is worse than Tomik against Depths and Lands, but way more applicable in more matchups. It's strong in the Delver matchup to shut down Wasteland, great against D&T, can deal with activated hate like Tormod's Crypt, so on and so forth.
I like Tomik for its uncounterability with Cavern on human, but WW is still hard to cast. I like that it's a creature that can be found with OUaT, and a 2/3 flyer is a nice body, but 2cc could possibly be too slow even in the Depths/Lands matchups...
I think I'm going to swap Tomik to Pithing Needle, tbh.