I'd like to give them credit for continuity but considering that they couldn't get Paige's birthday correct consistently (and her birthday was a really important part of plot because of the circumstances surrounding her birth), it could have been just dumb luck on their part.
Trying to make any kind of logical sense out of the changes to the canon in the latter seasons requires impossible mental contortions. That's part of why season 6 is so unwatchable for me. Magic school? Not believable. There were lots of situations prior to season 6 where magic school would have been utilized if it existed. They could have just made it so that it was a new entity. The cleaners? Ridiculous. Also should have been a new creation to account for inconsistency. I'm not sure why everything had to pre-exist. Given what happened to Prue and all of their reckless behavior after that (Leo orbed Paige in broad daylight in season 4 and in season 5, the manor disappeared and Cole went on a rampage using his powers in public), it would have made sense for evil and good to get together and try to prevent exposure.