The ending theme because the two dango that made the baby dango are slightly different colors than the two dango that are holding it(a pink and blue pair followed by a more reddish/orange and green pair). However this actually makes sense since Akio and Sanae end up raising Ushio instead of her real parents, Nagisa and Tomoya
For those who don't quite get the symbolism in the last part, the blue dango = Tomoya; the pink dango = Nagisa; the baby dango = Ushio; the orange dango = Sanae; and the green dango = Akio.
And the elderly dango with squinty eyes could well be Koumura-sensei.
Sanae tells Ushio that the only places where you can cry are in the bathroom and in her father's arms. Guess when was the first time she ever cried? Seconds after she was born...being held by Tomoya, because her mother was dying.
Nagisa's play, based on a story she only partially remembers, obviously parallels the plot of the girl and the garbage doll in the Illusionary World. In the end, Nagisa recalls that it ends with a song, which she improvs in her performance. Tomoya chides her that the song at the end of the story probably isn't "Big Dango Family". Actually, it is! It's hummed by the girl to the garbage doll as the Illusionary World is torn apart at the end of ~After Story~.
A common complaint I have heard about the ending parallels the ending to Nagisa's play. She sings a happy song at the end and Tomoya says it ruined the emotional buildup. And at the end of the series. All of the bad things that happened to Tomoya in AS was reversed. Some people didn't like that, because the series had been so sad up until that part and then since it was reversed it kind of ruined the emotional buildup according to some people. This is fridge Brilliance to me because I think that was intentional. Nagisa brushed off Tomoya's critique because in the end she wanted the play to end on a brighter note. And the creators wanted the series to end on a brighter note too. And for every person that didn't like that, there was a lot more that did. It was the correct choice since the series isn't trying to be sad, it's trying to be touching. We see the good and the bad of their lives and we become inspired even. Ending the series like that or with Nagisa and Ushio still dead would just make the series depressing rather than touching. It would make the series nihilistic rather than hopeful.
The reason Kotomi insists that people spell her name with three hiragana is that her name is a portmanteau of her parents: Koutaro and Mizue, who died in a plane crash. She wants to make sure to honor their memories.
Nagisa dying was a horrible blow for Akio and Sanae, but they were able to continue on knowing all their efforts and sacrifices allowed their daughter to be both happy, even if for a short time and also give birth to Ushio. But if the Episode 21 ending timeline continues as a spilt timeline, or in some fan's opinion, as the real ending, then they. Have. Lost. Everything. The daughter they raised is dead, their granddaughter is dead, their son-in-law is dead and they are too old to have another child. Everything that kept them going through their struggles and lives is gone. They will wake up in the morning to find that their dreams are totally shattered and have, more then anything, only each other, which as so many couples who have lost children can tell you, can simply not be enough sometimes.
Tomoya's father is just as done. He has now also had every single sacrifice he made become totally pointless. His son and granddaughter are dead and he will without a doubt descend right back into his depression, if not outright killing himself.
The World of episode 21 is simply a path of heartbreak and destruction, that is rightly solved by episode 22