EFCS vs Full Electronic Shutter (Silent Mode)

Several discussions are enumerating the pros and cons of using the Electronic Front Curtain Shutter (EFCS) instead of the Mechanical Shutter but very few are addressing the technological aspects differentiating EFCS and the Fully Electronic Shutter (Silent Mode in the Nikon jargon).

I have a few questions in regards to Electronic Front Curtain vs Fully Electronic shutters, whereas (at least in the way these are implemented in the Z7):

Common to both EFCS and Fully Electronic Shutter:

• Start of exposure, rather than being initiated by a quick cycle of the mechanical shutter closing and reopening, is initiated by resetting the sensor for a fresh new read.
• Sensor is read in about 1/15th of a second.
• Prone to some undesirable side effects, such as rolling shutter (subjects moving fast laterally), light flickering (non-continuous light sources) and some possible banding and other artefacts.

EFCS :

• Exposure ends when the mechanical shutter closes.
• Limited to 1/2000 of a second or longer exposure times.
• Flash can be used, at least up to the maximum sync speed of 1/200th of a second (I have not yet verified if we have HSS with Nikon speed lights set in Auto-FP mode).

Fully Electronic Shutter :

• Exposure ends when the sensor is read, which requires 1/15th of a second.
• There is no limitation of exposure time; works right up to 1/8000th of a second.
• Flash cannot be used.

My questions:

1. Why is flash utilization impossible when using the Fully Electronic Shutter. Since the exposure starts the same way for both EFCS and ES, what in the fact that the ES exposure ends the way it does, prevents a flash to be used. I fail to see there wouldn't be a moment during the ES exposure, where the sensor could be globally and properly exposed during the short flash burst when it pops...
2. Why is there this 1/2000th of a second limit on exposure time when using EFCS while there is none with ES ? What in the fact that EFCS ends its exposure the way it does prevents the utilization of shutter speeds right up to 1/8000th of a second as can be used with the Fully Electronic Shutter ? I cannot figure that, when using either the Mechanical or Fully Electronic Shutter the second curtain will successfully ends an 1/8000th of a second exposure while it will not for exposures initiated electronically when using EFCS (but not ES)...

Thanks for illuminating my lantern.

Michel