"Well, I suppose I'll just have to suck it up, my personal opinions aside. I set out to make with Arcueid a C implementation of Arc, not my own personal Arc dialect."
I too struggled with this. I wanted to change ar significantly in ways that I considered better. Then when I started the Nu project, it too made many incompatible changes to Arc 3.1. I still believe these ideas are good and improve Arc 3.1, but they're incompatible nonetheless.
But now my opinion is that any new language based on Arc should be cleanly separated. So I've started my work on Arubic (my own language based on Arc) as a separate project from Nu. To be more specific, it's one language implemented with Nu, the other one being Arc 3.1.
So now the Nu compiler should be very compatible with Arc 3.1, and any incompatible changes (like Arubic) will be implemented as something akin to a module or library. I think, ideally, an Arc implementation should be reasonably compatible with Arc 3.1, but still allow you to easily change it into a different language if you wish.