Outcome of journal as out of scope
One at a time:

Why are there 5 reviewers instead of the usual 2-3? There're many possibilities, the two most likely of which are 1) they routinely invite five reviewers because historically, not all of them will accept. This time they did. 2) Some of the first reviews received were poor (your description makes it seem like so, but I don't know the details) so they invited more reviewers.

Do you need to talk to the editor about how they came to an out-of-scope decision with major revision reviews? Probably not. The editor's decided that your paper is out of scope already, which is usually equivalent to a rejection. 

Does the editor suggesting you submit to a sister journal mean you have to go through the review process of that journal? Yes. The sister journal will have different requirements, and the editor might not even be a member of the editorial board of the sister journal.