Those wraith reapers are easily taken care of before they get into range. Three direct hits and they're destroyed.
Make sure you go into first person and rotate your torso so you're walking "backwards" towards the enemy, so if they start to get too close, you retreat quickly while firing at them.
There's no reason why you should get surrounded; if you destroy the turrets, you can have them chase you all around the empty canyon while you shoot at them. There shouldn't be enough enemies to where you can't destroy all of them before looping back around.
But the first person strategy isn't something I recommend; it's far easier and quicker to just build an Eagle APC and fly in to pick up the commandant.
The nuclear winter would be an interesting feature. Yet, I'm not sure about the "experience, economies of scale, etc." involving the manufacturing of nukes. If you want it to be a true "game-ending" condition, then simply make that nuke more destructive (and expensive).