question

>> What is responsible for activating the hub servers, if not matchmaking?

PlayFab provides an API RequestMultiplayerServer for your use when you want to request a server session without using PlayFab Matchmaking. It only accepts entity tokens of the title by default. So we would recommend you call this API on a custom lobby server or PlayFab CloudScript via multiplayer.RequestMultiplayerServer method, then allocate the IP addresses and Port numbers to players on your own.

In addition, please check this documentation to learn about managing standing-by servers before you call RequestMultiplayerServer API to allocate servers. For more concepts about PlayFab game server, please feel free to check our documentation.

>> How should a mission server be activated when 1 player wants to play solo (because matchmaking has a 2-player minimum)?

As the first answer said, you can call the API RequestMultiplayerServer on CloudScript then let clients call ExecuteEntityCloudScript API to execute the function to get the corresponding server details. You can refer to the following CloudScript function example.

// This an example of a function that calls a PlayFab Multiplayer API. The function is called using the 
// 'ExecuteEntityCloudScript' API (https://api.playfab.com/documentation/CloudScript/method/ExecuteEntityCloudScript).
handlers.requestMutiplayerServer = function (args, context) {
    var apiResult = multiplayer.RequestMultiplayerServer({
        "BuildId": "[YourBuildId]",
        "SessionId": "[YourCUstomSessionId]",
        "PreferredRegions": [
            //Choose the preferred regions in need.
            "EastUs",
            "WestUs"
        ]
    });
    return apiResult;
};