Bigfoot Is Real said:
Did they ever address the timeline issue with McGonagall, or is it just a retcon?
Probably a big mistake / retcon. But she said she'd been teaching at Hogwarts 39 years - the easy way to explain it off would be to say it wasn't continuous service... 17-20 years from 1910 to 1927 during Grindewald, then starts again in the late 1970s up to Harry's time... = 39 ish years.
That's not official or anything but would be a somewhat tidy way to explain it... Slughorn had a breach in service.
Did any of the Dumbledore pensieve or Tom Riddle diary flashbacks show McGonnagal at Hogwarts between 1930 and 1977?
Again, I hadn't even noticed that before (but duh! Now that it was pointed out) so... this would be A WAY to explain it.
Another would be similar... she taught therein the 1910-1930s; took time off; came back in 1956 and has been teaching there for 39 years the second time around...