Electronic – How to you measure low resistance accurately

The data sheet says that it has a resistance of 0.2 ohms with an analog signal of -5.5V to +5.5V. If the output of your variable potentiometer falls between that voltage range I see no reason why it wouldn't work. You are not really feeding "resistance" through it but current, no matter how small, but as long as your voltage and current are within the specs of the device it should work.

When you are "calibrating" the device you are sending a current through it and measuring the voltage drop to get at the resistance.

If it were me , I would not only measure C & tan delta but ESR as well and test to 10MHz.

You can use a scope rear sweep signal to drive an FM generator, but 50 Ohm is not the best source.

If you cant scrape a simple AC current source design to generate cap impedance as the output voltage, use a voltage source (ie. darlington emitter follower to drive the cap with an adjustment for DC offset +/-2 V and monitor current with a 1 Ohm shunt. The scope can show the DC bias on one channel and ac coupled envelope synced to scope sweep. This is a quick & dirty method. XY mode of V vs I is another method.