
I thought Mika knew they were dating (⊙…⊙ )

But, But... when Taichi said he loved someone he said that he loved that person [Naoto] since high school... and Mika seemed like she kinda guessed it was Naoto at the time?? Continuation error?

Could be. Because she said "the person Hase-kun has loved since high school...there's only one choice" and that can be interpreted as her guessing its Nao but her reaction is too off for that so I am guessing either she was gonna say something else and didn't even think it is Nao or as you sugguested a continuation error so that the author can use this character to cause more chaos.
Damn I love Ryou so much !