I don't know of the exact syntax off the top of my head, but you should be able to do it. Optionally, I've seen dealers use serial splitter software. That would allow both HomeSeer and MainLobby to communicate directly with the Elk.
This is certainly possible. You can use HomeSeer Events to trigger and send commands over.
Second, you can also use the Elk ethernet module and then have both HomeSeer and MLServer equally communicate with the ELK. I have had 5 separate systems communicate with the ELk. Works fine as long as you have the Elk Ethernet module.