1. Colin was in the DA, right? All the DA members showed up; they got "the call."

2. I think Dumbledore figured it must be Harry as soon as he figured out that there were Horocruxes. Also, Dumbledore says that because Harry doesn't fight his death, but goes to it with acceptance, it just kills the Crux of the matter (oh, pun totally intended) and not the Harry of it all.