You are correct that she might have been part of the MYP series, but that doesn't change the fact that she was not. The newest version of MOTU that many of the newer fans remember (since they didn't know Filmation MOTU), doesn't have She-Ra as a part of it. Even if they knew the filmation MOTU series, if they didn't bother to watch PoP, to them She-Ra wouldn't exist or need to exist. The original version doesn't have She-Ra as a part of it, outside of the sequel series (PoP). NA doesn't have She-Ra as a part of it. The only thing that has She-Ra as a part of it is the show created as a vehicle for her.
As for her inclusion in MOTUC, words mean nothing. It's Mattel's actions that matter. Mattel can say whatever they want but it means nothing until she's been created as a character or has been referenced in another character's bio. Once she's been referenced in a product, then she too will be a part of MOTUC. The same is true of any other MOTUC character that has yet to be created or referenced. For instance, Trapjaw isn't a part of MOTUC, although he very well might be at some point. I think our first inkling into her status will be Hordak's bio when it goes up.
When it comes time for a wide release, the She-Ra fans will matter even less. However, the same is true of the MOTU fans in general. A wide release will require a far larger fanbase than what can be provided by the currently existing hardcore fans, so the primary concern will necessarily be to entice the general populace, which is basically the situation that the movie faces as well.
I personally don't care much about her inclusion in the MOTUC stuff one way or another in and of itself. I don't collect toys anymore and my only interest in MOTUC is that I know people who are happy buying the stuff so I am happy that it exists. I don't really care much about what happens with the line on its own merits as long as my friends who collect it are happy. My only concern with She-Ra is her inclusion in a future movie. The thing is that I also strongly suspect that her movie status will be affected either positively or negatively by her MOTUC status. They could put her in one and not the other, but I think the MOTUC decision will affect the movie one too. Her inclusion in the movie is one of the few things that will cause me to not watch the movie at all so I am adamantly against it and won't in any way lessen that stance. It's not a matter for me where neutrality will suffice because it's an issue that will fundamentally affect my movie watching decisions. I want a MOTU movie that I will watch. If I am not given that, then I lose all interest in the movie.
As for his replying to the art.... I can't speak for why he does it, but to me that's just an appreciation of good art. For instance, I've replied to art for fan characters that I didn't necessarily like because the art itself was excellent. To me the quality of the art is aside from my views on a character, although I fully admit that it's a bonus if the art is of a character I love.