It looks like you answered somebody else's question.
How can you invite new members from discord?
Why would you go to discord to challenge other alliances? Then jump back to se where they are on ranking check their members hope to find their members on discord?
"basically you have all of this with Discord" what do you mean? only thing I get is newsletter what is completely different from one pinned page. Ok you could pin things in discord but for sake of one page its not worth it especially have chat here already.
Giving people ranks in discord is meaningless. There would be no connection to what they can or cant do in game.
In game there are only 2 ranks "no one" and "I can do everything guy"