What will the measuring electrical conductivity give you a metric of versus VWC?
EC can give you the solute concentration of the soil, but is trickier to interpret than VWC since the VWC can be a confounding variable.
What is the difference between the FDR method and the capacitance method of VWC measurement? Are they the same?
They are technically not the same. For the strictest technical accuracy, FDR makes a frequency scan and capacitance measures the charge time of a capacitor. From a practical standpoint there is little difference in the performance.
Why don't your soil moisture sensors measure with 1% accuracy like some others spec'd?
How well the sensors measure the dielectric permittivity of the soil; and
How well the calibration converts that measurement to water content.
Due to inherent variability in soil physical properties, it is impossible for a dielectric sensor to measure within 1% water content accuracy across a broad range of soil types, even with a perfect measurement of dielectric permittivity. Beware sensors that claim an unbelievable accuracy in all substrates.