I understand criticism, but I see you questioning Grim-Grin,
Intruder Alarm, and the zombies. These are put in to complement Phenax. In order to use the ability Phenax gives creatures, you need to have creatures in order to do it.
Undead Alchemist, Liliana, and Scarab God are the primary zombie engines.
Grim-Grin is used to get bigger to tap, Mill opponents, get creatures, and Mill more.
Intruder Alarm is there so that anytime our opponent plays a creature, all my creatures untap and I can Mill my opponents again. If a creature enters on my side, I can Mill more. Plenty of synergy between
Intruder Alarm and Phenax.
In regards to Zombies and Mill, consider the combo between Scarab God and Mill. Look at
Mindcrank and Scarab God or
Mindcrank and any of my Life Drain influences in the deck.
The deck may be a Mill deck, but it's meant to have multiple win cons on the side like LabJace, Life Drain, and Exiling cards.
I didn't feel confident running
Cabal Coffers or mono black ramp because it's not enough black to justify doing so. Remember, this is a Dimir, (mostly blue), deck and not mono black where those cards shine. I would otherwise need lots of black influence to make good use of them.
As for Tutors, I considered
Scheming Symmetry,
Demonic Tutor, and other tutors, but in the end, i otherwise opted to not include tutors because of the draw influences giving me enough cards in the deck anyhow and I don't need to search for any cards in particular.
As for Sultai, I don't believe Green does much in the way of Mill. I appreciate your suggestions, but none of them work with the direction I'm taking with the deck.