I hope not, I don't want to see alternate versions of old characters. Personally I hope that we never see that alternate world or Lucifer again. I like the idea that the Winchesters just saved their world but doomed another by adding a 2nd Lucifer to the mix (I feel like for the world to have gotten into that state Lucifer and Michael have to be going at it apocalypse style), it shows that their actions aren't without consequences. The reason I don't want Lucifer back is because he is far too powerful and they cant kill him and now they cant even cage him again, so short of God intervening or them releasing Michael any win they get is just gonna be some corny plot device.
Nope, I think that was just an inconsistency/retcon to make Lucifer seem more powerful. When
Lucifer killed Gabriel he used an angel blade, they are both archangels so there is no reason for Lucifer to have survived it. Also, surely Cas would have known that an angel blade was no good against Lucifer, personally I think that whole last scene was a mess.