1) Ichika
Yes: She's an actor, so she should be good at playing dumb. She's also one of the early proponents for Fuutarou. The oneesan personality in her dictates she defer personal gain for the benefit of the rest of the sisters, which would explain why she is not revealing herself.
No: There's no strong reason why she couldn't be the CF, except that it's strange that we see her clearly growing to love Fuutarou. I suspect that the real CF wouldn't do this, she would already love him but would clearly be holding back and playing dumb.
2) Nino
Yes: She's a schemer and highly intelligent in that sense. She'd have the cunning to openly play dumb with Fuutarou. Her most negative attititude with him among all sisters could be to make sure he maintains distance from her.
No: She's seen the photo of Fuutarou back in the day as a child, and reacted like she didn't know who he was, but yet thought he was cute. It's not the reaction you'd expect from a person who would recognize such a photo, unless she's really on her toes and trying to make you think that.
3) Miku
Yes: Well, I guess because she obviously likes Fuutarou.
No: Because she obviously likes Fuutarou. Someone trying to hide their past identity from Fuutarou is obviously not going to openly challenge for Fuutarou's affection. As a side note, Miku is the obvious candidate for heartbreak if this manga takes the non-harem ending route.
4) Yotsuba
Yes: Because among all the sisters, we know the least about what she is really thinking. She's the most helpful of all the sisters, which collaborates with the childhood scene where the CF bails Fuutarou from the police.
No: Well, there's no obvious reason why she couldn't be the CF except that maybe she's just too dumb/airheaded/simpleminded to pull it off.
5) Itsuki
Yes: Because she seems to be the main heroine among all the sisters and was the first girl introduced. By in large and far most commonly, the first girl introduced in harem mangas is the end game winner in non-harem endings.
No: Because she is obviously the type that is too honest and can't maintain a proper lie to pull off the act of acting dumb. Because she's the first girl introduced, and the too obvious candidate.
If you ask me, the most likely suspects to be the actual CF in this story are Ichika and Yotsuba. Nino and Itsuki have an outside chance and Miku is unlikely to be the CF.
Also, I happen to think that the doctor that treated Fuutarou at the hospital when he got sick was a relative of the sisters. Maybe even the dad?