Speaking for myself, I do not find that special to be out of continuity with the rest of the series. Rather, there was a persistent continuity error that carried throughout the run of Goof Troop in which several episodes treated Max and Goofy as though they had always lived in Spoonerville, seemingly ignoring/forgetting the events of the series premiere episode in which the two only first moved to Spoonerville. The Christmas special is among these particular episodes.
However, this kind of thing is to be expected of a show like Goof Troop, as it was common for such shows of that era to play fast and loose with continuity, as maintaining internal continuity was more of a suggestion rather than a standard for shows like this back then. Thus, while it is a continuity error, it is not one to be considered so irreconcilable as to outright declare these episode to be non-canon.
Frankly, I'd never go so far as to say that any episode of Goof Troop is non-canon, as that couldn't be further from the truth. All 78 episodes and the Christmas special were meant to be a part of Goof Troop, continuity hiccups and all.
I've since gone and edited the trivia note on the Christmas special's page to explain its contribution to the continuity error, as well as offering a potential fix for it by simply considering the Christmas special to just take place a year or so after "Forever Goof". After all, it's not like the series broadcast order or production order really make any kind of chronological sense, anyway.
And I'm surprised respective continuation was only a suggestion and not a rule.
Then it sounds like you haven't watched that many cartoons from the 90s, 80s, 70s, 60s and earlier.
Heck, even modern cartoons still do this. Example: One episode of Spongebob has Mr. Krabs say that the Krusty Krab employees don't get any breaks, while in another episode, Spongebob goes on his lunch break with Mr. Krabs telling him how much time he has.
Or in a more egregious case, the kid characters on Arthur have been in the same third grade class ever since that show first started in 1996, but while the technology of that show's world has changed with the times, the kids have never aged.
Or, hey, look at Pokemon. Ash has been ten years old since that show started in 1998, and is still ten years old to this day. No one ages in that show either. And yet continuity is key in that show.
I've done a LOT of editing on Disney's Goof Troop Wiki and it's kind of a drag going back and getting the same pictures for a page here. However,, there are some I didn't get a picture for before. And I created more episode pages. I'm a little disappointed that this wiki didn't do anything for the rest of the one-off characters as well as major and minor.
I'd be glad to add more screenshots from the shows, but I haven't yet subscribed to Disney+, and for some reason, Windows Media Player doesn't play DVDs anymore, so I can't take screenshots from the DVDs.
Well, I would think it would make the most sense for the iconic regular version of the song to be listed first. Plus, although the commercial you referred to does indeed have a copyright date of 1986, the toys probably weren't actually released (and therefore the commercial likely wasn't airing) until after the show started airing, unless the video in question did indeed come from a tape recorded in 1986.