In Elite Coop mode, if one of your ships dies it respawns, but so do all the enemies. I don't think destroyed allied Stations respawn though; if they are destroyed, tough, you have to battle on without them. Also, the enemies abilities are increased by the percentage set in the options. Your enemies can be made to be heavy hitting juggernauts if you set the percentages high, with small and medium vessels potentially on par with a single Light Cruiser or Scout. In essence, I don't think you can get away with going it alone and attacking enemy fleets independently in Elite Coop. You have to coordinate together to attack each enemy fleet, providing mutual support and making sure you fall back should you begin to lose a particular engagement. You'd have to utilise tactics such as firing waves of torpedoes, or having a combination of different vessels working together, like a missile cruiser working as long range support for a battleship.
In Invasion mode Siege, Single Front and Double front are the set ups for the sector. They determine which side of the sector enemies are attacking you from. I believe siege is any and all sides, double front is two sides (e.g. top and left) and single is just attacking from a single direction (e.g. from the left). I am not sure on the set up for the Elite Coop mode, but I assume that it will be similar to a double front or siege invasion.