Two questions regarding MP:
I don't play multiplayer games, but I'll try to answer your questions anyway.
My guess would be
Free
For
All.
2- How do you position your troops? If you keep a single SOD, the others can see where it is and attack you in the other side of your empire, if you spread your forces all around your land, they can attack you anywhere with a big SOD and you won't have time to group them to fight them off.
The defender has the advantage of quicker movement. This means that the defender can position his troops in several central regions and get to most positions in his empire fast, especially with 2 move units. Of course, this will become more troublesome with larger empires, but you should also be able to get more troops when your empire is larger.
Due to the faster movement, it should also be possible for the defender to get the first hit with collateral damage units. After several collateral damage hits, the defenders units should be able to pick off units from the SOD entering his territory with low losses. If the defender uses mounted units to do this, then he can hit the SOD and retreat back into safety while simultaneously severely hurting or destroying any siege units in the SOD. Collateral damage plus mounted flanking damage will make the SOD into a combination of units incapable of capturing a well defended city. A wounded SOD with few remaining siege units is ill equipped to capture cities.
The goal isn't necessarily to instantly defeat the SOD when it enters your territory. The goal is to make sure it cannot hurt you severely. Destroying it can happen later while it is trying to run away.
Diplomacy is probably even more important in multiplayer games. You can't single handedly defeat all the human opponents at once. You can't hope to win when surrounded by enemies from the start of the game. So you need to make deals with several of the human players, maybe to take out another player together or for other reasons. If multiple human players combine their forces against you, then you're in trouble.
The defender can also use spies (or late game air reconnaissance) to see where the enemy is concentrating its troops. Scouting the (potential) enemies is important.
You should also make sure that the cultural buffer (the tiles of your culture between you and your enemy) is as thick as possible. If units can get to a border city very quickly, then that city is very vulnerable. If you fear a seaborne invasion, then scouting the possible approach areas can be extremely important to react to attacks quickly enough.
An increased healing rate should also help the defender when the war takes longer. The faster movement will help your units retreat from enemy SOD's in your territory to fight another day and the greater healing rate will mean that surviving units can join another battle quickly.