Need Simple, Easy-to-Use "Goof Proof" Audio Headshell Recommendation

Basical no, you're looking for the wrong solution. The head shell should not be threaded as every other cart is different, many actually have a threaded hole so no nuts. Carts cantilever/stylus are also never/rarely perfectly aligned with the carts body so proper alignment could not be accomplished if the head shell was not flexible. Also, different carts have different overhang, etc. You need flexibility in the head shell for different carts, unless you have a specific cart/tonearm set up that only uses that cart i.e. B&O Beograms.  

As much pain as it is to set up, once done you really don’t need to touch it again until it’s worn or changed. If you have more than one cart buy another head shell.  

One note, if it is taking you 2 hours to set up a cart I'd suggest you invest in better tools ie alignment gauge etc. Good tools will make this job much easier and faster. I can set up a cart in about 30 mins max, other than the listening test for VTA.  

Keep in mind that adjustable azimuth, which everyone seems to demand, carries with it the opportunity to maladjust azimuth. In the hands of an amateur, that can happen believe it or not. This results in aberrant stylus wear and distortion.

I otherwise agree with the above; you want slots that permit minute adjustment of overhang but not because cartridges demand it, instead because tonearms demand it and different alignment algorithms demand different overhang distances.

what you want, what the entire industry should give us, is threaded inserts in any cartridge body, so two fasteners from the top are all that is needed, so easy to set overhang and null points alignment.

if every headshell had azimuth adjustability, that would be nice

open sides makes it easy to deal with the wires

I prefer removable headshells, and I like how Micro Seiki 505 arms have an adjustable fitting for easy azimuth, another industry standard I wish existed, then no need to adjust the headshell itself.

VTA, easy smooth arm height while playing/listening exists: no tools, loosen locking lever, rotate up/dn, tighten all by hand: examples:

micro seiki

acos lustre gst-801 (I learned about mine here when I joined in 2019)

technics EPA-B500 Adjustable Base also has removable straight and s arm wands

52mm overhang? 52mm must be specific for something else, overhang is the distance from the spindle to the stylus tip centerlines, the longest I have ever set is 15mm, I’ve never seen a protractor beyond that.

square to the headshell, not relevant, when aligned to the two null points, may be slightly off-square in the headshell.

Do you have the alignment gauge that came with the SL1200GR? 

If your cart has threads, just put both screws in and tighten until the cart will not slide easily in the headshell slots. Then using the gauge, line up the stylus tip as per Technics instructions. I wear a cheap pair of magnified glasses when doing this.

When you have it lined up, tighten one side a little more so that the cart can be rotated if need be. I then either measure the gap between the screw head and the end of the headshell slot or use a small screwdriver or other object as a feeler gauge. Rotate the cart if needed so the slot size is the same as the first. Tighten the other screw and recheck with the shim or measure. 

For carts that are not threaded, I just drop one screw in the headshell slot and slightly thread a nut on. Put the cart against the screw and headshell and tighten it up, it will now be easier to get the second screw and nut on. Do the same as above.

I have checked my results with other methods and this has worked every time for me.

your idea, sort of

concealed threaded holes

search, hifishark, headshell adjustable

thanks to you, I know about this one