I see no improvement in printed quality scanning a photo over 300PPI regardless of print size.
Yes, because 300dpi captures just about all the information in a print, and it's only worth going higher if you see an improvement. You MIGHT see an improvement if a specially fine-grained printing paper had been used with a very sharp print.
Scanning negs to the highest quality requires >4000dpi; prints need a bit over 300dpi. This reflects the fact that printing paper wasn't designed to be scanned or otherwise enlarged; it was designed to be looked at.