1. If the HDD is being accessed (woken up) multiple times, then leave it spinning because of wear from spinning down and spinning back up. Cursed WD Green HDD line had high failure rate because of that, (forced 8 seconds idle timer, spin down, park head, Windows requests something 2 seconds later, spin back up, repeat until drive dead). Most HDDs are 5W or less during spinning idle so its not like its sucking up power.
2. Like Kilrah mentioned
3. Have not heard anything like that, SSDs have their own built-in protections so probably not an issue