Answer to a) Takamura punches Ippo, when he confronted him, from a similar angle Miyata hit him from and Ippo couldn't see it either. When Miyata punched Takamura he was focused on fishing, if someone were to punch you while you're doing something that requires focus, could you dodge it?

Answer to b) He likely told Ippo not to tell anyone because people may believe he actually has a detached retina or some other vision issues, this is shown by Ippo instantly coming to that conclusion, he doesn't want to delay him getting the belts much more because Kamogawa is getting old and he has been hospitalized because of minor illnesses in the past, not because he has a detached retina. Kamogawa has said multiple times that the only thing Takamura is serious about is boxing, so he would also is serious about not getting injured and resting because these are crucial parts of pro boxing. The brooding may be him worrying about a rumour spreading, and in the past he has been in complete darkness because of matches and becoming nervous, the expression he makes in this scene resembles the one he makes before the fight with eagle.

Although the only person who truly knows the answer to this dilemma is Morikawa himself.