1. It depends. Just use the one that's necessary Unless the tank is fully geared, and always use Benefic II if you end up in Stone Vigil. They actually want you to cast the regular benefic which can proc an instant crit version of benefic II.
2. You really shouldn't worry about getting all of the unique seals during progression. Just try to get as many different seals as possible and use divination followed by a lord or lady of crowns and stack them together for a 8-10 % buff.
3. Yeah, totally. The cooldown is like really long, so it's not like a loss if you do it.
4. Diurnal is like way easier since it's a regen healer, so you don't have to memorise incoming tank busters. Shield healing is about preventing damage beforehand so it does have higher risks and rewards, IMO