The planet is still ok if the nid infestation is full...but the ability for your shrines reduces alot. (not so sure what the shrines do other than allow more activations of your forcefield....maybe faster life regen)

I think you have to keep at least one planet from full infestation. If all 3 planets get infested, its gg for you. So try not to lose too many battles or reduce the infestations when u can.
I prefer the scouts over the assault as the assaults seem to die too quickly and easily even with my best armour and seals on them. Once i land i can't even drop a stun grenade before they die off. Thus i prefer to send in scouts to throw stuns or snipe the artillery units before finishing them off with my dev and tactical squads. Terminator armor for the tac and dev teams help alot.
Question to ppl who have finished the campaign: