He also has a very small speaking part at the beginning of the Halloween special Mickey's House of Villains, but it's done with footage recycled from "The Stolen Cartoons".
Additionally, he makes non-speaking minor appearances in five other episodes: "Big Bad Wolf Daddy", "Timon and Pumbaa", "Where's Minnie?" (while he is seen speaking at the end of this one, wishing Mickey a Happy Birthday alongside the rest of the staff member characters, he is not voiced by Jason Marsden, instead merely having his lips animated with no voice coming out while everyone else's voices are heard over him, drowning out his voice from being audible), "Dining Goofy", and "Mickey vs. Shelby".
And as for what your wrote here, yes, I am aware of that comic. Heh, I'm the one who started the thread about it. All it shows is what might be considered an analogue of what she may look like. Yes, there are theories about her fate, but that's all they are, theories. Nothing official has ever been given about her. All that's ever been addressed about her in any official capacity, in the Goof Troop episode "Date with Destiny", is that Goofy is Max's only parent. Anything about his mother was completely skirted around and avoided being mentioned. She's just a complete and total enigma, her identity unexplored and her fate undisclosed.
Like I mentioned in the other thread, I've got Goof Troop/A Goofy Movie media out the wazoo, and am currently working on a gigantic, ambitious continuity-based project in which I'm trying to create a timeline that maps out the full chronology of the Goof Troop universe, reordering the episodes into a workable chronological order, as well as including every comic, book, game, movie, etc. that I can fit into this timeline, as well as trying incorporate seven other shows into this shared universe: DuckTales, Chip 'n Dale Rescue Rangers, TaleSpin, Darkwing Duck, Quack Pack, Mickey Mouse Works (albeit, as performed works rather than representing the characters' real lives), and House of Mouse, plus Mickey's Once Upon a Christmas (but NOT Mickey's Twice Upon a Christmas since that one contradicts too much to fit into this greater universe).
I've currently completed all of the Goof Troop parts of this project, with only a basic generalized idea for everything else. So, if you were ever wondering when each series took place, here's a basic rundown:
TaleSpin - estimated late 1930s/early 1940s (I'm hoping for a timeframe that includes 1938-1941)
Mickey's Once Upon a Christmas - the first segment has to be at Christmas 1985, the second segment could be any Christmas within the mid-1980s (I haven't pinned down a precise year for this one yet), the third segment is even more vague on its year so it too is yet to be determined, and the ending medley scene could be at Christmas 1986 or 1987
DuckTales - I'm aiming for this to be from June 1986 to April 1990, after having re-watched every episode and the movie (but still making my way through additional media like comics and such)
Chip 'n Dale Rescue Rangers - haven't gotten to this one yet, but I'm hoping for this to be from the late 1980s to the early 1990s
Darkwing Duck - still making my way through this one too, but I'm hoping for this one to begin sometime in 1991 (or in post-April 1990 if necessary)
Goof Troop - got this one completely reordered and nailed down as being from June 5, 1991 to July 31, 1994, with every episode and other story given specific real world dates
The Spirit of Mickey - including this video due to its inclusion of Max, and placing it on July 2, 1994, during a large gap I ended up with between the final two Goof Troop episodes in my new order
A Goofy Movie - got this nailed down as taking place from June 2-17, 1995
A few other pieces of minor Goof Troop media end up being placed after A Goofy Movie during the intervening years (1995-1998) leading up to An Extremely Goofy Movie. It is also during this time that some, but not all, of the Mickey Mouse Works shorts would be produced by Mickey, Minnie, Donald, Goofy, etc. to be shown a few years later at the House of Mouse, but I'm getting ahead of myself
Quack Pack - I'm currently looking at a 1995-1996 placement for this one, but I need to think about this one a little more
An Extremely Goofy Movie - no specific dates nailed down for this one yet, but it must come from about August 1998 to May 1999
House of Mouse - 1999 at the earliest since Max has to be at least 18 to even be a parking valet, with the episode "Max's New Car" needing to come early enough in a new episode order to place it before Max's 20th birthday since he's still a teenager in that episode
There may be more things to be included in this timeline, but for now, this is the short version of what I've got so far.