Interesting approach, I’ll think more about it. Although, having worked with it, I consider the opt-out semantic quite natural and enjoyable to work with, and it works for most cases.
I tend to think that bringing amount on the stack is too error prone to bring in without bignums, especially since you can start with 4-byte amounts and aggregate up to 5-byte ones.
If my conjecture is true that “equality checks is all you need” beyond CCV, you avoid doing math altogether.