Regarding the ACARS messages, if they are genuine. The window sensor messages refer to sensors which detect the temperature of the window and are required in order for the WHC to control the amount of current required to maintain the window at optimum temperature for anti ice and/or defog. The messages do not indicate that power is lost to the windows or the WHC, although failure of the sensor will result in the WHC removing power from the affected window. For whatever reason the RH sliding and fixed windows lost the sensor signal.
The WHCs live quite far forward on the aircraft with WHC 2 on the RHS. Certainly not below the lavs or even the FWD galley to be honest.
While I don't want to speculate. I believe that the Egyptian authorities have been a little quick to state that terror is the most likely cause. It's looking more like an uncontained event. As to why there was no communication? Has anyone considered that the second flight crew member wasn't in the flight deck at the time (rest break a little before TOD?) or was somehow incapacitated?