They use a standard AR 15 barrel nut (w/o the delta ring). Use a quality nut - the rail slides over the nut and ALL the little cogs that stick out have to be straight, Troy makes a nice one that only a couple bucks more but it's beefy.
Troy does not provide that info.
Troy does not provide the weight for each individual SOCC 5.56mm Low Profile BattleRail. I would recommend reaching out to them directly for more information.
These hand guards are designed to mount up with a mil spec ar15 barrel nut. You will only need the one barrel nut used to mount your barrel to the upper.
Yes it should fit.
Yes this will fit M&P 15