Is it permissible to say "Ameen" after Al-Fatiha while catching the Imam in Rukoo‘, or must one bow without saying "Ameen" if saying it is recommended? And does the prayer become invalid by a slight delay behind the Imam to say "Ameen" and then catching him in Rukoo‘? What is the ruling on saying "Ameen" after the Imam recites Al-Fatiha if the Imam does not say "Ameen", and is it permissible to say "Ameen" before him if he does say it?

Saying "Ameen" after Al-Fatiha is recommended (mustahabb), and a slight delay behind the Imam to say "Ameen" is permissible. If the Imam omits saying "Ameen," it is still recommended for the congregant (ma'mum) to say it, and this is not considered a breach of following the Imam, because the deviation is minor. This is similar to the Imam omitting the sitting for rest (jalsat al-istirahah), in which case the congregant may perform it, because the deviation involved is minor.