Regarding the achievements, don't worry : it's meant to be that way. You can't unlock achievements while using mods. Regarding Multiplayer, it's very odd as you absolutely can use mods in multiplayer. You are supposed to activate it before creating lobby, once it's done you can select it.
I would advise you to try again with another mod that you edited a bit less as it's possible that the game files have been edited too much and now multiplayer has a hard time with it.