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!
"Let me be very clear about this, Mister Chekov. Do not go to the Delta Vega system. Ever. Do not let any ship you are serving on go there. Do not let any ship rumored to be considering it go there. You are to commit mutiny and premeditated murder if that's what it takes to prevent any captain from going there. Is that clear?
"Yes, I know you weren't on the Enterprise when it happened, but that didn't stop you with the Ceti Alpha system."