> a) can I sub in the 500 safely for the 470?
Certainly.
> What does it accomplish?
Reduces radio reception/transmission.
> b) are silver mica overkill/unnecesary for this project?
Yes (but what the heck).
Your tin roof is loose, buzzes in gusty wind. Put a rock on it. Big enough to damp the buzz, not big enough to dent the roof, but you don't fret whether it is a 470mm rock or a 500mm rock, granite or shale. I'm sure Apple's builders use ceramic; in this value ceramic is a near-perfect cap. They used 470 because that's how we number things since the 1960s; Silver Mica has been hanging around since the 1950s.
If you build an amp with a BRITE switch, I'd use the SM there. But mostly cuz it looks good, I suspect it sounds the same as ceramic.