Out of sheer curiosity, what was the reasoning behind hardcoding envelope 1 to always control the amp?

This is a product of the way that voice allocations work: because Envelope 1 is always connected to the amplifier, we know that when it has reached the end of its Release stage the voice is most definitely silent and can be turned off.

In theory, this may be something that could relaxed in the next version: the modular routing parts of Aspect are getting a big overhaul, and I suspect allowing a little more flexibility here is possible...