they need Michael and Lu to have a vessel to end apocalypse once and for all...maybe Michael isn't exactly enthusiastic about killing his brother and Lu's avoiding his brother as much as possible..they are so like Dean and Sam..
but again that makes no sense, cos Lu's supposed to be the most powerful angel, the most beautiful..so killing him, outside of God, i don't think anyone can..or whatever..

I don't think they can because the first time Michael used his sword on Lu it sent him down to hell instead of killing him.

If they could just kill each other then they wouldn't need Dean and Sam to be the vessels. I think the vessels help to focus their powers or something when they're on Earth.

no idea!!maybe they need the meat-suit in order to kill..at this point i don't know!

i recon they could, maybe just fallen against angels, cause not many angels would have that entention unless theyve been to earth and seen that kinda destruction and most of them usually have vesels on their trip

i have no idea

But no, they probably do need a vessel.