If that had been done correctly, that could've been neat. The person should have just highlighted the high points and peaks but not the seams and shut lines. It's too busy.
I've used a bamboo skewer or toothpick (sometimes a q-tip) dipped in Novus and seems to work well. Just don't rub so hard you burn through the foil or flatten the detail.
Same here. I always do it by eye. I keep some mixed colors and reuse them on other kits if there's enough, but most of the time it's a per model thing.