Those should not be your concerns when all you have is a game idea. You should be focused on making the game. And at no point should be you saying, "I am making this game for the sole purpose of getting it onto Armor." There are tons of great sites out there and if you make a great game, you'll want to get the most for it and get it everywhere.
However, if you're thinking about a multiplayer game, I'd look into Player.IO; their infrastructure is great and relatively cheap.
And... to answer your stated questions: If your multiplayer game is going to be on Armor, you'll either need to integrate our API for SSO or have an in-game registration that is seamless. For the second, we do not have our own platform currency... as such, almost all games on Armor with payments use their own integration from a 3rd party (such as from the companies you listed).
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