1. Unless DHX or Hasbro says otherwise, it's canon and should be used by anyone who wishes to use such info.
2. Look to my point of her not going all out and Chrysalis was getting supercharged by Shining Armor's love for Candence. Before and during the fight, Chrysalis was scared shit of Celestia and even credited her victory through siphoning her love for Shining Armor. The vines were machinations of Discord, one of the few characters who is definitely stronger than her, and they caught her off guard.
3. You still have no evidence to support Celestia is a slow flyer, while I can power scale it off from her sister who has shown speed feats and Rainbow Dash as the 'Crystal Heart book', which DHX has stated numerous times is canon, that Alicorns have the traits of all ponies so a stat increase across the board for say Twilight when she became an Alicorn, let alone the much longer lived Celestia.
4. A. Comics are canon and can be used so long as DHX doesn't contradict it.
B. Celestia has been viewed as more powerful than her sister, and is further supported in the comics when evil Celestia nearly killed her own evil sister without going into her super mode while Luna needed to be in NMM mode and a Celestia refusing to fight her just to be able to fight her effectively.
C. She isn't a Mary Sue considering that she doesn't do much in the show and whenever she does she usually gets her ass kicked due to Worf effect, relegated to a supporting role to the mane 6 or had her sister tagging along in her fights, and has shown to be not absolute as there are a couple of characters stronger than her in Tirek when fed, Discord, and even Twilight has the potential to exceed her. In fact, the show needs to show her being stronger because she's been suffering the Worf effect for too long.