A good chunk of what makes Highlander* decks viable, beyond the two power cards currently pushing the archetype in the first place, is that a lot of different cards kinda mostly do the same thing anyway (or at least accomplish the same thing because there's usually more than one way to skin a cat in HS). Occasionally they're worse, Shadowbolt isn't a great card by any means for example, but most of the time they get the job done equally well. When they don't should be made up for by times where nonstandard cards actually pay off, as you have more overall options available to you, and the extreme power level of Reno/Kazakus. Plus Hearthstone decks aren't that large anyway and, specific to Warlock, Life Tap is OP.
I actually didn't hit a single mirror match, I would have left out Rat/MCT if I could've known, but you need at least one of Rat/MCT combo or Leeroy/Faceless combo not to punt the mirror. (The latter to beat Jaraxxus, the former to pull the opponent's combo so you can win via Jaraxxus yourself, or pull Kazakus and win on the board.) I've never liked playing with MCT, it's not really a condition you want fulfilled and its unreliable when you really need it. Rat is actually not terrible on its own though; fast decks are unlikely to have the meat in hand that makes the Battlecry regretable and empty their hand onto the board fast enough that sometimes it's just a ridiculously undercosted card anyway, slow decks it disrupts by pulling out situational minions being held back. Alchemist is my pet ladder card but it's not really useful beyond sniping nonbasic totems and I only hit two Shaman (one of which dunked me at 10-0 with a nut draw), although I did set up a sick 4/8 Mountain Giant versus the final boss Priest.