Mark said he would never come back because of the way he was treated at the end.

Lucifer wasn't trapped in Hell because of Crowley he was trapped on an alternate earth.

Two things.

Jesus is not part God, he is fully God and the second member of the Trinity. He was fully God and fully man when He was on earth.

The second is Mark himself explicitly said he would never come back to Supernatural because of how they did him wrong.

Let's not forget these quotes from 11 and 9

There's no such thing as an ordinary human. Do you know, in 900 years of time and space, I've never met anyone who wasn't important before.

Eleven 

"Rose, there's a man alive in the world who wasn't alive before. An ordinary man: that's the most important thing in creation. The whole world's different because he's alive!"

Nine

I would also assume that the higher settings would cause it to overheat if used longterm in a firefight, just like if you are using full auto, you can't just fire nonstop for too long without melting the barrel