I should qualify that thick roots are not necessarily penalizing due to reverse flow, but with a typical helicopter the thick root would likely see closer to zero flow on the retreating side (and subsequent lower drag). They basically add to the overall rotorhead effective area.
Also, ever wonder why Sikorsky never got the rotor fairings to work? Those were one of the trumpeted key differentiators to making X2 an improvement over XH59A.
Symmetrical airfoils help with profile drag to a degree, but the drag is primarily driven by overall thickness and chord length.
And yes, TPP is driven by flapping at high speeds. Im sure Sikorsky would gloss over the fact that their speed is a "dash" straight line capability, as even with the X2 I would be very wary of trying a 1G pullup or bank at speed. Let's hope they dont want to try NOE flying or SAM evasion!
On any rigid rotor the flapping and coning values are going to be relatively low (hence the higher loads). That said, can you build a blade that can accommodate these loads at a large rotor radius that:
Doesnt weigh an astronimical amount
Is stiff enough to keep flapping under control with rotor spacing that keeps hub drag low enough to hit high speeds
Does not overload the hub attachment/components with beam/chord loads and mast with hub moment
Is dynamically tuneable
The answer to this is yes for small-ish GWs. Current composite materials allow for a solution to this equation, but not for the elevated loads of a 24,000 lb FVL-M demonstrator.