Shred Memory's purpose is twofold. It's graveyard removal, yes, but more importantly, it's a 2-drop tutor as it can transmute, which can fetch pretty much any of my cards that I intend to win the game with. This deck capitalizes on greedy gameplay of opponents-- so if you ramp a lot, grab Price of Progress. Swinging with something big or casting a big damage spell? Deflecting Palm. Feel like a boardwipe is coming? Grab Boros Charm. Opponent swinging with a bunch of creatures? Get Batwing Brume or Rakdos Charm. Need a dramatic game changing sorcery? Get Burning Wish. Got Master of Cruelties in hand? Get Key to the City.
The deck specializes in having answers whenever needed by capitalizing on card draw and keeping it flowing with the Monarch token, and eventually making yourself invulnerable to most things with Solitary Confinement, preferably combined with Necropotence or Phyrexian Arena to give you stuff to pitch to it.
Sunforger is also a key card as it allows me to tutor up and cast instants (which I have a LOT of) at instant speed. So with the Tutor suite, the Shred Memory, Burning Wish, and Sunforger, I'm packing a toolbox deck full of answers to be grabbed at any time needed.
As far as Dread and Teysa go, I've considered them. Teysa's actually in my "sideboard" of sorts (read: handful of cards I have in the back of my deck box to experiment with as needed). The only real reason I'm not running her is that I wanted to keep my deck capped at 5 mana, giving it a tight, versatile curve. It doesn't play HUGE threats because not only are those expensive and really hurt to have destroyed (with no real way of reanimating them), they draw the ire of the table and encourage opponents to attack you when the goal is to play the politics game instead. You're not playing any real tricks with this deck. You're just setting up a system where it's just not rational or reasonable to attack me, because either it will tax you or you'll be punished with losing life, creatures, permanents, that kind of thing. The goal is to make the deck feel like a stax deck when you're targeting me, and a group hug deck when you're targeting opponents. That's where cards like Duelist's Heritage come in-- I'll help you out by giving one of your dudes double strike if you don't swing at me. (Or, I'll give you double strike when you're swinging at me and Deflecting Palm it back at you). Lots of versatility.
I'm really excited for some of the new C17 cards. Mathas, one of the vampire commanders, lets you put bounty counters on creatures which encourage opponents to use resources to kill other opponents' stuff for me-- and everyone but the targeted opponent is rewarded! Disrupt Decorum literally forces everyone to attack each other while leaving me alone, and Teferi's Protection lets me phase everything out and protect me from big threats from a turn. Very cool stuff.
And yes, I do want to build the other Marchesa too, but that's a deck for a later day.