Very good write up and discussion. I also think Mentor is to powerful to not be restricted. The answers to Mentor have minimal impact as where vs Shops (when Lodestone was not restricted) 1 Pulverize could destroy the entire Shops gameplan. Where Dread of Night/Illness in the Ranks can be answered by Misstep, bounced with Repeal and have no impact on the rest of Mentors gameplan. I dont think Gush should be the target of the restriction, it should be Mentor.
Also, lets say we restrict Mentor, does this just open the door for Pyromancer to be too powerful? I actually dont think so. The card relys ONLY on Instants and Sorcery spells. Mentor gains value off of every spell in the deck besides lands with pumping itself and generating other creatures so the card advantage is not equivalent from Pyromancer to Mentor.
What I could see happening with Mentor restriction would be a solid meta of...
Shops (making a comeback)
Pyromancer/Delver (Delver could be playable again)
Big U decks
All of these decks play very well off of each other. It has diversity and your SBs can be evened out in a great way to strategize against the field.
Doesn't that look more healthy than MENTOR.FORMAT? I dont know, maybe its just me.