Spock is making hurry a priority by his odd choices. Why strive to reach orbit immediately? That only makes Scotty work under unnecessary pressure, possibly leading to mistakes. Why drain the phasers so soon? This could be done at any point, just before the chosen time of departure. And firing at the natives would not drain the phasers, unless the cavemen arrived in the thousands - "Omega Glory" teaches us that much.
I'm surprised by your stance here. It looks pretty clear to me, as ssosmcin
pointed out, that they have a very limited time frame. Getting the shuttle fueled up to take off is high priority. On top of this, they have the angry Sasquatch natives to deal with, who seem to sense when they're wandering about. Having people outside making a burial is like throwing chum in the ocean and thinking sharks might not come.
Every time a phaser is fired, they suffer drainage. And, they don't seem particularly effective against the natives, except to frighten them only momentarily. They never mention phaser settings, so we don't know what setting they used (probably too early in the show before they established this detail), but a logical guess would be stun at first, then kill when that wasn't effective. Citing one line from "Omega Glory" is not credible, as Tracey was deranged at that point anyway. "Thousands" is ambiguous in this context, given his state of mind. In fact, Spock says there were "several hundred Yang bodies" that they discovered near several discharged phaser packs. Who knows precisely how many were killed per power pack, but it's definitely not in the thousands. So, the power would need to be conserved. As it was, they barely had enough to get into orbit and landing was theorized to be very rough, if Spock hadn't jettisoned the fuel and ignited it.