really two isn't bad though. I always have one main map and alternate the second map depending on what everyone wants to play or if I need certain bosses for Tek
you can always ftp in and download the save files. easiest way to transfer is if you have a cluster setup you can just upload to obelisk. Other than that I would say you would need to make a backup, upload everything to the obelisk, change the map, go to the obelisk and download everything. You would need to make sure allow tribute downloads is allowed.
some of the files are already there if you check your ftp server there is a genesis folder. I would imagine some got loaded while others will still be patched when the update goes live. much like when you pre-load a game you buy there is always a day 1 patch.
I would try having them add it through the steam browser not the in-game one. Or you can always go on the server and have them choose friends from the server list and it will show them the one your on.
You have to do enablecheats youradminpassword If it doesn't work then you don't have an admin password set in your Ini. Or like Geoffry mentioned make the AllowedCheaterSteamIDs.txt file. I find that is much easier so you don't have to enter your password every time you connect.