Deprecate on the grounds that it is slow in decimal.py or the
InvalidOperation issue?

I think pure integer arithmetic with the decimal type always requires
attention from the user, since in many functions one has to check for
Rounded/Inexact in order to get meaningful results. Here one has to
check for InvalidOperation, so I don't think it's a big deal. (This is
also the reason that I don't find the inconsistencies that you listed
particularly bothersome.)

If slowness is the reason: If any C module makes it into Python, this
would not be an issue any longer.