Tell your W you are not prepared to discuss this with her right now (as in tonight).
I thought you had all this info already. DO NOT AGREE TO ANYTHING WITH HER UNTIL YOU TALK TO AN ATTNY.
And your W's point is not valid. Divorce changes everything including things for the kids. If she chooses to change the arrangement then things for your son will change. And to accommodate the 50/50 he will go to daycare and she will get LESS support so you can pay for daycare. DO NOT SAY THIS - I AM GIVING YOU IDEAS ON HOW TO TAKE ONE LINE ITEM AND PARLAY THEM TO SOMETHING ELSE.
Talk to an attny and find out HOW to fight for 50/50 custody. I AM BEGGING YOU NOT TO TALK TO HER UNTIL OU FIND THIS OUT FROM AN ATTNY THAT KNOWS EXACTLY HOW TO DO THIS.