The only people to successfully add either of those things to a mod are in the Platinum Team, and I don't think they have the time to explain it all. They can probably also guarantee you that they don't remember, not even as a group, everything they had to do. Adding multiplayer (or maybe a chatroom) to a mod is very involved, so the vast majority of people don't consider it worth attempting. If you can't implement, say, a marble selector from scratch, you have quite a lot to learn before you'd be able to understand half of a tutorial for making a chatroom.
I think a much better alternative to making a fancy mod is making a level pack for PlatinumQuest. That way you can use lots more features, even multiplayer, and you still (at least in singleplayer) have the ability to add many new features to levels. I found getting features to work in multiplayer daunting, so I haven't made more than a few simple experiments for that.
The Platinum Team has made PQ's code public, which helps a lot with adding features to custom levels:
github.com/PlatinumTeam/PlatinumQuest
It would be a good idea not to make multiple topics for the same question; it's kinda spammy.