It doesn't really matter about the rachni queen I believe. If she's from the first or the 3rd. If you get mordin to sing in two he'll sing at the end of his whole "mission"

To get the closest "perfect" game you can, you'll have to do the multiplayer. If your galactic readiness is high enough it makes your effectiveness in single player higher. They are currently recording voices for the "closure" DLC so that might be out soon. And it'll be free!