Keep it simple. Interview gf? GM? WHY?
So when the kids are with dad, if he wants to get GF to pick up kids OR baby sit OR leave them with Granny that is his call and it is a positive because it shows the kids have extended family that care about them. Sure your angle is he doesn't even look after them other's do.... But that sort of talk gets you labelled as having a negative attitude.
Your argument? 4 or 5 is adequate. More than enough 50/50 in a few yrs sure. But right now everyone is adjusting to this new situation and the kids need a home base. AND right now dad isn't really managing the time he has as he relyies upon you to pick up one of the kids once a week even though it contradicts the orders AND you really need orders that mean you're only being relied upon by dad in emergencies NOT on a week by week basis.
Don't bad mouth dad, GF or granny. Chances are dad will bad mouth you. Let him. Makes him look bad because he is focussed on you when he should be focused on the kids. But dont you make the same mistake.