SME 20/2 Finger Lift Hum

Unless I have completely misunderstood what is going on here (and that is entirely possible), I don't quite understand why you are so exercised over it. If insulating the lift has cured the problem, I especially do not get it. I would think of that as a "permanent solution" and go on from there to enjoy the music.

For example, my dedicated MM phono stage will put a low level buzz on my speakers, if I don't dress the umbilical cord from its power supply so as to be clear of the AC cord. So I taped it up to stay away from the AC cord. I consider that "case closed", but the reality is not unlike the one you describe; I used a bandaid to fix a problem. So what?

If I understand the situation, this was NOT an issue with a prior cartridge, but, it is an issue with the Lyra Argo. If that is the case, it may be the case that the Lyra's aluminum body is part of the conductive pathway. In that case, you may be able to do something with plastic screws, but, depending on how conductive is the headshell and the body of of the Lyra, you may also have to put some kind of barrier between the cartridge and the body. Keep in mind that the Lyra body is designed to make a very tight physical contact with the headshell in order to effectively drain vibrational energy from the cartridge into the arm for dissipation by the arm. That "insulation" might compromise that objective.

In short, I would go with Lewm and consider insulation of the armlift as a permanent solution.

It's certainly a workable solution since it's what I am doing and when actually playing a record it sounds fantastic, but I will still be experimenting until I find a 'permanent' solution. The concern about the tight and rigid coupling with the Lyra to the headshell being compromised is a concern of mine as well if nylon screws are used. Thanks for all the advice. We'll see where this goes.

If the SME V arm tube is anodised, then this explains why it doesn't hum when touched- anodising gives a non conducting surface.
The question here is what finger lift are you using. It would seem to me when you are touching the arm lift or cartridge bolts you are creating a ground loop.
If my theory is correct then finding an arm lift with a non conductive coating is the answer - and gets rid of the plastic problem. Another solution you might find more elegant would be to put some heat shrink on the arm lift - just the part you touch. You can get different colours including clear.
You could also try wearing rubber soled shoes or put a rubber mat in front of the tt to stand on to break the earth loop.

I have an SME V arm and have never installed the finger lift. I don't want another piece of metal that could vibrate a tiny amount between the headshell and bolts near this sensitive part of the arm and cartridge. I don't know if this sounds better or even different as I have never compared, and perhaps it is just a silly precaution, but rather than take the chance, I just left it off. My dealer also suggested this when I bought the table years ago. It looks more elegant anyway, IMO and I don't miss the convenience of the finger lift. This should solve your problem.