A1. By @MrDaniels :
I would say that would be rather incongruous.
A2. By @VanVeen :
I've seen pictures of it with charcoal suits. It looks weird to me. Most people probably wouldn't notice or care, but it's wrong.
A3. By @Leaves :
I dunno what the etiquette is in the US, but dressing "above" your guests is considered inappropriate in most parts of Europe. I would guess the same goes over there in North America. If you know a lot of your friends will not have the means to acquire a tux then I'd opt for an "informal dress" dress code.
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