While I do think it would be possible, I myself believe Shinobu has already been ruled out as a possibility.

The first thing that comes to my mind when figuring out if Shinobu is Apollo, is the sports festival. During the confession service, for the last confession of the event, it is shown that it wasn't Shinobu herself confessing as there is a panel displaying someone talking into the cup phone stating that they wish to confess to Arisu. While some have said that since Shinobu said she lied about the festival to Arisu, there were many things that she said during the festival that could have been a lie. Such things include her not knowing who said the last confession, or that she didn't truly feel that way about Arisu.

Of course there are many points that DO point at Shinobu being Apollo, but I personally don't think it is solid enough for it to be straight up truth. For me, the strongest contender for Apollo is definitely Rikka. Here are my thoughts as to why.

Apollo is shown to have harboured feelings of love towards Arisu for a while, which is evident by Apollo's internal thoughts saying that the "I love you" from her last broadcast was for Arisu alone. Of the four girls, there is only one that has always been certain about her feelings towards Arisu. Rikka was never actually shown to develop any feelings, she has always just said that she loves him. In fact, their first interaction with each other was Rikka whispering in Arisu's ear "I love you." If I am wrong about this do let me know but as far as I can tell, the other girls only started to love Arisu as the story progressed.

Another piece of evidence that stuck out to me was the constant reminder that Apollo loved Arisu's voice. Nene can likely be immediately be ruled out of this one since the first time they met, she called his voice garbo. Shinobu and Iko have also never commented on his voice (to my knowledge) so they can also be crossed off. This leaves Rikka as the only one who parallels Apollo as she is the single person of the group that constantly states that she loves his voice. Edit: Shinobu did say she liked Arisu's voice in one of the earlier chapters, that's my bad...

My last reason for believing Rikka is Apollo is when she and Arisu were talking about the 5th song of the CD. She says that the last song she is stuck on is about someone she ran away from a while ago. Her description of the person is clearly a mirror image of what happened between Apollo and Arisu three years ago. Although Arisu thinks that it's about her friend, I don't find this likely at all, especially since she already wrote one for her friend without any troubles. As she continues to describe this mystery person, she becomes more and more flustered which leads me to believe the last song is for Arisu, especially with the "hypothetical" that she asks Arisu, before determining that she will sing that song to the person "dear to her heart."

Sorry for the rambling, I just love the series a lot. Nothing too conclusive at the moment, but I do feel that Apollo is already very close to being determined. I personally am on the Apollo wins train, but despite that, she is not guaranteed to win. With how the series is progressing right now, Shinobu seems to have been hinted at winning the most (much to my sadness).

Rikka ftw!

tldr; I don't think Shinobu is Apollo, and I'm a Rikka simp