One possible reason could be public nuisance. If he was performing acapella very loudly in inappropriate places or at inappropriate times, disturbing the peace, he could get arrested.
Perhaps the main characters are two friends who start an acapella group together and as they work on their music, their relationship deepens into love. Another possibility is that it's a story about a music teacher and a student who are brought together by their shared love for acapella, and this connection blossoms into a love story.
Well, an acapella love story could be about a love that is expressed through acapella music. Maybe it's a story of two singers in an acapella group who fall in love while making music together.
I'm not sure specifically which 'acapella boy' arrest story you are referring to. There could be many incidents involving boys who are into acapella and get into trouble with the law for various reasons like theft, disorderly conduct etc. Without more details, it's hard to tell the exact story.
In general, the Evangelion mangas are considered good. They expand on the anime's universe, providing more depth and character development. However, it depends on personal preferences as some might find the style or pacing not to their liking.