After you have repaired this, how are you going to convince yourself that it is safe to use (for instruments and yourself)?
What tests will you do?
First I will keep the ground clip installed, which will have a good ground connection up to the handle and where the 1 MOhm resistor is grounded.
Installing new cables will help a lot for safety, since the old one is partially broken
May be I will test the tube for high voltage leakage but I am not convinced that this is needed.
Since I have to build a a guide for the resistor anyways, I might build a Teflon tube, to add some additional level of safety.
You have any further ideas?
Well, I just tested the 999 MOhm resistor with my Keithley 2450 SMU
With 100V source voltage, the resistance is measured kind of stable to 994.6 MOhm
So, I will assume that the main resistor is working well.