Arc Touch Mouse SE won't reconnect on 10.10, mods to blame?
I can now confirm that the issue is a bug in OS X's Bluetooth implementation. The mouse connects just fine in 10.10.0, I hear it connects fine in 10.10.1, but has the issues I've been describing in 10.10.2.

I can also confirm that the issue has been fixed in the latest OS X 10.10.3 public beta.