1) Is there a difference of opinion among shi’a marjas on the topic of music or is there a consensus?

2) What are some examples of music that is not played in gatherings of vice and evil?

3) Couldn’t classical music also be played in a gathering to slow-dance to?

1- There is a difference of opinion. Some understand that all/most types of music are haram. Some believe only if it’s the type that’s suitable to entertainment gatherings then it’s haram.

2- That varies from one society to another. In your society, if there is a type or music that is generally not played in concerts or entertainment gatherings (like some types of classical music or therapeutic music or war music) then it would be permissible.

3- Yes it definitely can. But is it typical of such gatherings? Not really. Usually in such gatherings such music is not played. Other types of music is played. So if classical music is generally not typical of or suitable for such entertainment gatherings, then it’s permissible