Won't hurt - if they don't need it, they won't take it up. and its not like grit goes bad...
Neither mulch nor dirt makes good grit. if there's a source of tiny rock in your dirt, then ?maybe?. Mine seem to make a go at the recycled concrete base (basically, crushed concrete) around my barn...
^^^ that.
and in some environments, you can get a white, vaguely moldy (actually yeasty) growth across the top. Its called "kahm", and is not harmful. Arguably, its beneficial in that it prevents more dangerous things from colonizing your feed, just not desirous in food making.
Really depends...
Wet food is fine, anytime. Unless its in danger of freezing solid before they can eat it. Be careful with the blueberries and bananas - treats should not exceed 10% of diet by weight, you risk unbalancing their dietary intake.