The root cause identified as a propellant valve lag is not an official report only a twit – a clear PR. Elon Musk is ingenious communicator he makes understandable explanations of everything. Form the other perspective the faulty hardware appear unusually frequently in experimental vehicles of SpaceX having in mind lack of such problems in highly successful Falcon 9 (as expendable LV). So the narrative about some valve lag should not be taken very serious.
So the answer would be: Yes, CRS-6 landing failed because of 'a conceptual flaw'. The lost of efficiency aerodynamic controls at low speed even is not root cause but contributed to repetitive problem.
Unfortunately, at terminal descent the efficiency any aerodynamic control surface diminish dramatically because of low airspeed (under 60 m/s they are inadequate). While aerodynamic controls had lost their efficiency, the aerodynamic disturbances caused by surface proximity – turbulence and wind gradient remain significant due the large platform area of the booster and legs deployed (ref picture).
The graphs of angular movements tracked by Rhett Allain from 13 April crash video shows a classic pattern of dynamically unstable system – an oscillations with exponentially increasing amplitude. A very similar picture may be reconstructed from foggy night video of the first barge hard landing.
The primary cause of two very similarly failures should be different. Tha main suspect is single ended attitude control at final approach.
The rocket flight control by single point vectoring the trust of its main engine have been developed prior WW II mainly by Goddard and Von Braun. One of problems of such arrangement is challenging requirement to maintain two variables (rotation and translation) by single effector (vectored trust). The metaphor of balancing a stick (in the middle of a wind storm) is in fact a physical model of the system.
typical angular/lateral stabilization [from 1990 Russian textbook]
Acceptable solution was refined during the development of intercontinental missiles by applying of emerging those days automatic control theory . The basic on idea is to separate control channels by its frequency response. The attitude stabilization has priority and guidance system reacts faster to correct the angular position (high frequency). The lateral movements are corrected relatively slowly (low frequency) adjusting the flight path during the ascent progress. The modern flight computers apply adaptive and predictive algorithms to adjust guidance response during the changing flight conditions. Nevertheless, the basic principle remains the same.
A typical frequency responses of large ‘well-tuned’ rocket are shown in table below (flying normally upward). Note the slow reaction on center of mass movements.
Center of mass movements 0.01 -:- 0.03 Hz
Angular movements 0.1 -:- 0.3 Hz
Fuel slosh 0.5 -:- 1.5 Hz
Body elastic modes 2-:- 15 Hz
The try to use the same trick flying the large rocket backward has limited success to the moment. It works mostly on light vehicles, out of the dense atmosphere, or flying slowly.
In contrast, of orbital launch, the pinpoint landing imposed a strict limitations on lateral drift. Thus, the lateral maneuvering became an ultimate priority and forced a faster response. Because of close frequency the lateral feedback interfere with the pitch/yaw channel. Figuratively speaking, the guidance computer is in vacillation between contradictive priorities: attitude stability and horizontal deviation.
Flying backward the rocket performed divert maneuver in a little pervert way - by purposely destabilizing the booster tilting it in desired direction and then erecting it back. That tricky maneuver inserts large swing what is hardly to dump by rocket optimized for very different flight profile.
Parallel parking problem
Another problem of the vehicle trying landing back by same means using during the ascent is reversing the position of gimbaled trust controls. The controls become in front regarding flight detection during the descent. Steering characteristics are changed dramatically this way. Even fully predictable movement of a car is les versatile (and more stable) driving forward with front steering wheels. A parallel parking driving forward is simple analogy of what vertical takeoff vertical landing rocket is trying to do. The top of tall cylindrical body lag the guidance intentions due the aerodynamic dumping.
Back to sixties
Contemporary launch vehicles are strictly optimized for axial stress at vertical launch. Although any attempt to use a system developed and refined for specific purpose (vertical ascent) for completely different one (vertical descent) will certainly confront once more the basic problems already solved during primary use development. To enable for pinpoint vertical landing the rockets designed to fly against target or to launch a spacecrafts will need to rethink and possibly re-solve the basic problems of propulsive flight to meet the new emerged requirements.
An article on G-FOLD project. You can find how the single ended control works on practice.
FLIGHT TESTING OF TRAJECTORIES COMPUTED BY G-FOLD:
FUEL OPTIMAL LARGE DIVERT GUIDANCE ALGORITHM FOR