[Adding admins]

This is a good writeup for adding admins to your server, https://pingperfect.com/index.php/knowledgebase/108/Eco--Admin-Commands-How-to-admin.html. There should be more data in that Users.eco file than just the MaxSlots value, did you fire up the server and login to make sure it was running first (maybe the rest of the data didn't get populated properly).

My Users.eco file has a ton in it that I didn't touch other than the admin section.

To your example though, it should probably look something more like this:

{
    "MaxSlots": "10",

    "Admins": {
        "$id": "4",
        "System.String": {
            "$type": "System.Collections.Generic.List`1[[System.String, mscorlib]], mscorlib",
            "$values": [
                 "76561198020218658",
                 "76561198093830148"
              ]
           }

      }
}

There is a backup setting in the control panel, but I think the one of the config files might have a setting for server save interval, I'm still new to this and haven't played with that yet. I usually just manually save from the ingame admin commands before restarting the server for something.

[Can not figure out adding mods to my server!]

I'm in the same boat, I add mods to the mods folder per the instructions of the various mods and the server doesn't start up properly.

Adding mods appears to through some item/access errors in the logs, maybe a permissions issue on one or more of the server files?