What the show did with Cozy Glow as a villain was interesting to me, but then they took it too far and locked he in stone. She deserves a second chance, and as Sawtooth Waves said in his video Reforming Cozy Glow, Diamond Tiara is the key. It would take a while for Cozy to be be ready to face Princess Twilight again after her reformation, but when she finally does it’s because she finds Grogar’s(missing as of then) Bell. She returns the bell to Princess Twilight and Princess Starlight Glimmer and Diamond Tiara practically begs them to forgive her. Of course, Twilight being Twilight, and Starlight having gone through a similar situation forgave her.
 
This is a prequel to Cozy’s New Family and Happens during the time of the main plot-line within the Wish On Starlight story.