Hi
I've been trying something similar some years ago. The somewhat disappointing thing about multiplay with debug executable is that if you change the game in any meaningful manner (like adding another weapon to a plane, change the round of fire, etc.) then it causes a de-synchronization: all the players suddenly get connection errors and then disconnect.
As for spawning objects, if you are the host, then you can simply use game.spawnObject <name> to get objects to spawn on your position. However, if you are a client, then using game.spawnObject will spawn the object on the host's coordinates instead of yours. It's quite funny to spawn tanks this way when the host is flying a plane. Instant crash

Clients can use Game.SpawnObjectAt <name> <position> to spawn things on given coordinates x/y/z. You still have to know your approximate position though.
I once dreamed of starting a live modding competition, where people would join a debug server and the be given like five minutes to code themselves a vehicle of doom. Then they would fight in those vehicles, the last one standing wins. Of course there would have to be lots of rules for the modders, for example forbidding the use of ObjectTemplate.HasArmor 0. Unfortunately, the whole thing is impossible because of the de-synchronization 'bug' I described above.