For example HTTP/2 and HTTP/3 require HTTPS. While technically HTTPS is redundant, .onion sites should avoid requiring browsers to add special casing for them due to their low popularity compared to regular web sites.

It would give you a certificate chain which may authenticate the onion service as being operated as who it purports to. Of course, depending on context, a certificate that is useful for that purpose might itself be too much if an information leak

Yes, but browsers moan if you connect to a website without https, no matter if it's on localhost or an onion service.