Yes And well no, not always. I came across him on the street. = always = I ran into him on the street. = I met him on the street I ran into him on the street. - has two possible interpretations (1) I met him on the street. (2) I physically collided with him on the street. So sometimes is the same. Most often, as collisions are rare.

Yes

The only difference could be if the speaker actually physically came into contact with someone. In that case "I ran into him on the street" would take on a new meaning. But we often say we "ran" into someone with meaning we physically made contact with them but you have to watch the context in which it was used.