You can use the createVehicle and camCreate commands to make things spawn in the middle of a mission. Or you can create them in the mission editor, hide them on an offshore island and use setPos to move them into the action where and when you want.
With the player it's more difficult. Learn about eventHandlers - in particular the eventhandlers "hit" and "killed". I'm not sure if you can bring a player unit back to life with setdammage 0 if he has been killed but it might be possible if you're quick enough. You can then use setPos to "respawn" the player back at base or whereever you like. You might need to use a titlecut blackout and black to cover it though, otherwise it might look odd.