I think running a big, non-blue Planeswalker is probably the best idea for this slot. It makes it harder to interact with on the opponent's side. I like the new Gideon in this role, though personally I have been testing Ob-Nixilis, Reignited. He performs well in these grindier matchups by letting you draw more cards Painful Truths style, or just comes in and nukes a Goyf. Plus if you do get him down on a stalled board, he's quick to ultimate and WILL win the game if that happens. Gideon is a nice trump to decks other than Shardless BUG, because they can amass enough creatures to attack him profitably. The other upside of Gideon is that you can pop him for an Emblem to take a dump on opposing Sulfur Elementals, Izzet Staticasters, Dread of Nights, and other such nonsense. Other contenders would include stuff like Elspeth, Sun's Champion, but I think 5 mana is already flirting with unplayability in most cases. I don't like Sigarda here because it's just so hard on the mana, but definitely see the appeal as it's a total bomb that takes over the board. Planeswalkers are great threats because they play nice with Monastery Mentor as well. There's even an argument for a BW Sorin (either one), but he doesn't do much by himself without some friends to tag along.
Berserking Now -- How are you liking the extra Surgical? It seems like a very reasonable inclusion, especially on MTGO where there is a lot of Snapcaster and graveyard action going on.
theMonster -- I like the idea of the Delver version, but I think you want to capitalize a bit more on the cheaper/free spells. Maybe going up another Gitaxian Probe? The problem with this deck in the Delver shell is that it's harder to take advantage of the Mentor to apply pressure early. You need a Deathrite to get it going faster. The advantage of playing the current slower build is that once you can stick a Mentor and cast a few spells to get value, even if they kill it, the Monks you make are enough to win the game by themselves a lot of times. I also notice a distinct lack of Daze in your list, so I would recommending shoeing in those as well. In addition, you might want to consider going to the all dual land manabase. While it does open you up to Blood Moon, you will always have the right mana, which is way more important for decks that establish themselves in the early game like what you're trying to do. Besides, 4-color Delver decks do a fine job without basics already, and they're not too far from what you're trying to accomplish. I would recommend cross-referencing those lists with the Mentor ones you find here until you can come to a happy medium. Cheers though, and happy brewing!

