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Patrick,
I get the sense that your builder is working outside his/her comfort zone. You might want to read this article to help get him/her back on track: https://www.greenbuildingadvisor.com/article/how-to-design-a-wall
FWIW. ZIP R is a good product when you want high performance, but it is a premium product.
In Zone 5, you need at least R7.5 on the outside of a 2x6 wall. You could reach this with 2 inches of EPS, which would be easier to install and detail than your proposed 3 to 4 inches of rockwool. (Many posters mention that keeping rockwool coplaner can be challenging.) If it were me, I would consider using plywood or regular ZIP as the sheathing and sourcing reclaimed EPS for the exterior. If I could not convince the builder to buy-in with this approach, I would talk to him/her about a double-stud wall (with cellulose and adjusting the footprint to recoup the lost space, if possible) or using the Bonfiglioli method to address thermal bridging (see https://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge).
Thank you sir!