I've been an executive in software for over 20 years, and bottom line: a colossal mess-up the likes of which Khadgar and related realms during the failed server merges (namely, character deletions, rollbacks, and de-leveling -- for weeks, apparently) tends to happen under circumstances of limited experience and personnel on the issue. I suspect that Blizzard likely has staff shortages to deal with the issue right now, and given how unprecedented this is (arguably a month-or-more of rollback impact and credited subscription time), we could be looking at ongoing performance issues for some time to come.
I do NOT suspect that the failed server merge has a material impact on the rollout of the new expansion or its pre-patch, but assuredly, it can't be helping matters, especially if they are short-staffed.