Voldemort just cares about himself. He doesn't care if there are casualties in the process of getting what he wants. He killed Snape so that the elder wand would work for him, so why not have Barty Crouch Jr. take up a dangerous job? He was a loyal Death Eater, but not a powerful one And Umbridge wasn't one of his servants.
"she's evil enough to be one" "yes, but the world isn't split into good people and death eaters."
He needed someone inside Hogwarts to get the stone, and all he had at the time was Quirrle, so there was no alternative there.
Snape was originally just supposed to teach at Hogwarts. There was no job specified. Snape just preferred DADA.
As for Amycus Carrow, it's the same thing as with barty crouch Jr. They were expendable. Also, Voldemort had near complete control of Hogwarts at that point, so there wasn't really anything to worry about.