That's not necessarily true. All of my AR-pattern rifles have a floating hand guard, and as such they are only attached at one end. A quad-rail attached in such a fashion would absolutely be sturdy enough to be used for all sorts of purposes like laser aiming modules and whatnot. So... The only thing "wrong" is exposing the gas tube. IMO.

No. It would work fine. Handguard are floating all the time. Attached at the back of the upper only. Actually how must nice rifles are set up. Ideally, it should cover the tube though.