This honestly makes no sense. It's not exactly Madoka's fault, or even a significant problem for other Megucas, that Homura is able to visit UKG. She was there, none of the rest are. She's the only link to that conceptual space and she destroyed the doorway, it's fine.
BTW, on the matter of Ai, someone else's thoughts from a chan board:
"
"Homurakami/Ultimate Homura is Ai and was the new principle born from Madoka and Homura's joint wish to never forget one another and to reunite one day. When Madoka gave Homura her ribbons and prayed for a miracle at the beginning of the story, that "miracle" granted was their bond and took the form of Ai. In the manga, she is refered to as "I" and it is meant as the embodiment of Akemi Homura's emotions for Madoka.
Homura thought the sheild was responsible for her remembering Madoka and she was afraid to destroy it. She chooses to destroy sheild in order to save the world. It was the "miracle" that bonded them though, so she got an improved bow and kept her bond to Madoka.
That soul gem in (pic related) is actually a good visual for ai yo. When Homura allowed her soul gem to burst, Ai had already filled her soul/heart and that allowed Homura to emerge as a new existence.
It isn't stated that Madoka knows about the existence of Ai, but her mutual desire for never forgetting and being reunited was required to create Ai."
So pretty much Homura's unique emotion reserved for Madoka in Rebellion was the result of the "miracle" that occured between the two during the naked space hug, Gen Urobuchi has also said in the past that this was in fact the "true" miracle of the series. That Madoka finally realized Homura had been fighting for her sake. Rebellion reversed this miracle by returning Madoka to not understanding Homura and Homura continuing to suffer for her sake."
