What does emergeny healing mean then? Panic spam healing touch?If the whole lifebloom heal is going to be effective and you haven't got anything particularly important to do for the next 3 GCDs, you should let the lifebloom expire. If not, refresh it with a nourish if it's not going to overheal or with a cast lifebloom if it would.
You can reach the haste breakpoint easily with 463 heroic items.
There's a lot of differences between dungeons and raids. Mainly the fact that you don't need to emergency heal or heal inefficiently so much. If you're going to do 25 man then sure, practice on the RF. 10mans are however a bit different to the 25mans.
Overhealing is always bad, but inevitable. Overhealing is basically just wasted mana.
And as for keeping as many hots up as possible, depends. You shouldn't blanket rejuv, but try to keep WG on cooldown and use rejuv when it's going to be effective. Just use your abilities wisely. Heal when necessary, trying to keep yourself down to as little as possible "everything else that WG and SM" is always going to be a good start for healing properly, but don't be afraid to use other spells. Overhealing and emergencyhealing are the 2 things you should avoid the most.