Even despite the rather broad gap in belief between, say, the PCUSA and OPC? Does not the OPC consider PCUSA to be in apostasy?
Catholic religious orders are distinctions among particular groups of Catholics, not divisions; but the divisions between denominations - real divisions that the denominational bodies themselves seem to agree exist - seem to me (an admitted outsider) to be a completely different thing altogether.
No. Both denominations believe in the Triune God; both hold the Scriptures as their rule of faith and practice; and as esquirette pointed out, both subscribe to the Westminster Confession of Faith.
Does not the Catholic Church consider the SPXX to be in apostasy? Given time I'm sure I could come up with more. There are good Protestant groups and there are bad, just the same as in the Catholic Church.