As far as Mitchell being killed off, was he really? We see him fall into the grave and a huge rock falls on top of it. It looked to me like the rock just covered the top of the grave and didn't necessarily crush Mitchell to death.
They could've done a follow up where Mitchell returns and this time he's REALLY pissed off!
Really! The guy took a full phaser rifle blast to the chest and it didn't even bother him, I doubt a rock, no matter how big, would have really hurt him. Let's assume Dehner's force lightning weakened him enough for the rock to win.