I hope this will answer your questions:
Unfortunately, the character "Bela" stole and sold the colt in the episode titled "Dream A Little Dream of Me", so they weren't able to use it to shoot the invisible dogs known as "hellhounds" that used Dean as a chew toy.
After Sam was stabbed and killed by Jake, Dean made a deal with the cross-roads demon to bring Sam back to life. But, in return, Dean would have to forfeit his own life in one year's time. He he was told that if he tried to welch on the deal, Sam would die. And Dean, being the protective big brother, could not and would not let his baby brother die in his place.