It seems unlikely that the aircraft was not properly deiced as the fluid was frozen all over the aircraft, I.e. fuselage, both wings etc.
According to the deicing company they were providing us 100%, would there be any water added to that? Your theory of the possibility of an excess amount of water in the fluid seems to make sense until I realized that given the conditions even 50/50 would have been more than adequate. We were airborne within 15 minutes of the start of holdover.
And yes the flight control checks were obviously carried out after the procedure was complete, and without issue. There was no issue with the controls. But the fluid was frozen all over both wings and on the fuselage. We used the camera and could not see the colour of the top of our fuselage. The rear half of the top and sides of the fuselage was covered with frozen fluid.