To answer your questions...
1. can electrical users in the aircraft accept such variable frequencies?
Yes, in three ways. Firstly, they could accept frequency-wild AC with no caveats. The amount of power delivered per second is the same regardless of frequency. Such items are usually purely resistive (so wing anti-ice, window heating, electrical heaters etc). Secondly, they could actually run on 28v DC, where a switching-type power supply rectifies the AC to DC. These typically don't care about the frequency of AC power supplied. Finally, you can rectify into DC and then invert back into AC for heavy duty motor loads (a synchronous motor on frequency wild AC is a bit useless). The 787 does this for the air compressors / CACs / engine starting / etc etc.
We have power outlets that clearly mention that they're on a fixed frequency of 60Hz
See point three above - they'll be rectified and inverted.
Secondly, what's the benefit of a VFG vs an IDG or alternator/generator with a separate CSD
Weight saving. You can also use a VFG as a starter motor in some instances.