Im not really following what you are doing, but it sounds like you need to use Animation Curves and actually change state or the animation playing based on the weapon being carried.
Instead of saying “additive this”, “additive that”
Just output the position of the bone as the x/y/z of the curve, then when that curve is present use it to drive the local bone position of the bone you want to drive with it.
I’d go with complete new values dropping the additive part, so that you don’t get confused and that you can just replace instead of doing additions/math.