1. I do plan on having a single chat lobby, so hopefully we'll be ok there.
2. I think the first version will require only the base files, for exactly the reasons you mention. We'll allow mods in a later release. The server will pick the options.
3. In game chat will go to everyone, if you want to talk privately you have to use a courier

4. We won't have a ton of maps at first, but we'll keep releasing them.