How to measure/compensate wire's stray capacitance
Several points of confusion here.

To measure capacitor leakage, stray capacitance doesn't matter.  You're looking for the current the cap draws in DC steady state.

Guarding (surrounding a trace with actively driven guard traces) is a means of dealing with non-perfect insulators and nulling out the leakage currents that would otherwise run thru them.  You recognize that the material has some finite resistance.  You arrange for a signal that is actively driven to the same voltage as the sensitive signal to be on the other side of that resistance.  Since the voltage accross that resistance is now zero, there will be no current flowing thru it, therefore making it not matter.

Guarding is not accomplished by using a twisted pair.  That is for a whole other set of issues.  Basically a twisted pair tries to make each signal couple equally to the environment, thereby making any noise picked up from that environment common mode.  This is not relevant to your problem of measuring the leakage of a capacitor.

Stray capacitance is very hard to predict.  Sometimes you use a shield to trade of capacitance to some unknown source with a higher capacitance to something that doesn't carry noise, like ground.