@GHitchHiker - In regards to the speed of the deck, I actually have slowly been cutting cards that make the deck faster in lieu of more consistency and have found the builds closer to the Legacy version to be putting up much better results. I feel that the deck is fast enough to handle most things well, its worst matchups being Tezzeret-based control decks with Thoughtcast and Dredge thus far. Dredge is a weird one, though, I keep going back and forth on how I feel about the MU and how much I want the third Terminus MD for that MU.
As for the basic Mountain, I had one in the sideboard for a long time and hated that it ate a slot there. I opted to eschew it, moving it to the maindeck over one of the cut moxen. Cutting it for some other basic is likely fine, but it is important that it is a basic for the Shops matchup, as well as any hatebears or similar decks you may encounter.
Why playing just 1 MM in the list when the most important card to get the lock engine going is SDT.?
Besides that, the deck has to fight all the cards that can harm your plan being mainly: other MM, Claim, REB effects, and all the efficiency of 1 CC spells.
I would try:
3 Mental Mistep (maybe 4)
2 Mana Drain
You also need to protect your SDT when replaying it after putting it on top of the library to counter 1cc spells in case you don't have one on top. Plus helps to reveal 1CC vs. other 1CC spells with Counterbalance. Plus they protect your Mentors from STP, and Bolts.
All in all, the deck seems pretty strong and well positioned in the current meta.
I have been testing with @Winglerw28 a fair amount lately, and I have been very surprised by this deck. I thought it wouldn't be nearly as good as it is. I have come to the decision that it feels very similar to landstill, in that, if miracles doesn't lose in the early game, it is probably going to win the game. I was surprised by how good counterbalance specifically was, and Terminus.
@PeAcH Misstep has been an odd card for me, as I've found I want to cut more the more I play the deck. I agree that Top is good to protect, but I've found just having the full four and extra cantrips to be better since Misstep is worse in the later game (with Counterbalance performing its role).
@PeAcH also, I am extraordinarily conservative in the way I play Mentor (perhaps too much so at times), so protecting it isn't something I feel is necessary since I often can just find the redundant copies or can win with Jace. I most often win through Jace ultimate.