Incredibly detailed info! Thanks for taking the time to post this. It is definitely appreciated.
I've owned several Zenit cameras (E, 12 XP, 3M) FED 5 and Zorkis S and 4. These are hard to beat, some parts are interchangeable across whole family. Funky breed, but as long, as the job is done, I won't complain. I prefer bottom-loading models, but it's a matter of taste.
Take notes like mad.
Take a note which lens was on which camera (note SN). Note which camera was in which case. I hope your cameras have modern 1/4'' tripod threads BTW, but it may be not the case, old models come with wider 3/8'' threads.
Unscrew the lens, take off the rear plate. Clean the camera thoroughly. Check for loose screws. Clean the pressure plate carefully - this is the only place where a stubborn metal particle will scratch the film.
Look how the camera behaves. Cock the shutter, change speeds, fire a couple of times. The shutter should travel really quickly. Sprocket wheel should turn. You should see shutter curtains moving as you wind the knob.
Shutter curtains should be black/dark gray cloth, even, flat, looking like new, with black metal tapes on the ends. Blow the dust out of it, don't ever dare to touch it. If the curtains are looking dirty, funky or cracked, check out if there are no pinholes in it.
If one or both curtains travel slowly, the camera will need a service. Don't believe in flexing "stalled grease" - if it's dense now, it will only get denser later. It ay behave better just to get worse afterwards or in the cold. (Part of my own testing usually takes place in seriously sub-freezing conditions, but that's just me). CLA, however, can be done with a Swiss Army Knife literally, and a borked camera is an opportunity here.
Check if slow speeds are working normally. Go through all shutter times, though 1s will show what's going on really. Play a lot and watch carefully for anything suspicious, like non-parallel curtains.
Check if self-timer is OK: cock the shutter, cock the timer, press the button above the timer lever. If timer is malfunctioning, you may simply avoid using it.
Check if the rewind collar works. You should be able to rotate the collar around the shutter, it should go down a bit. To wind the film later, use the rewind knob (rotating it will make the sprocket wheel turn freely). The shutter button also rotates (for shooting with "T" time, when "B" time is set). After rewinding the film, the shutter button may stuck in "locked" position.
Set the shortest time. Fire against something bright. Be sure to watch carefully if the light goes through on all the edges of a frame. If the light gets through at the edges, you're done with visual tests of the body. Clean it all one more time.
Now the lens. Jupiter lenses have no clicking f-stops. Be sure the f-stop scale rotates all the way and that it's aligned properly (= doesn't go outside of the range, isn't shifted etc.)
Clean the lenses carefully, but really well. You want to do this once only, so do this like you will only do this once. Blow, brush, microfiber with rubbing alcohol, in this order. Check for scratches, internal dust and fungus. You'll have to live with whatever there will be.
Check if the lenses operate smoothly through full focus range - often the grease (I believe they've used thick animal lard for the lenses and self-timer, hence the funky smell) polymerizes and becomes waxy. I'm a lousy, lazy person, so I lube them without disassembling - a single drop or twoo between the coupling ring (the part sticking out on the back of the lens as you change the focus) and the outer barrel gets the job done. People here will hate me for this, but it doesn't oil up glass or blades, at least in my case. The focus should be easier to move than the aperture ring - it's not a rule, but it's good that way. Loose aperture ring is a disaster.
Screw the lens back, cover it with the lid.
Check if the rangefinder works. There's an green-ish or yellow-ish image patch in the middle of the viewfinder with the imposed image moving together with a movement of a coupler sticking out a bit from the lens mount. Two images should become one when a given thing is in focus. Check the rangefinder with the lens set for infinity (any distant object will work), forget the close range for now. If there's a slight vertical misalignment - live with it, it needs the top of a camera to be disassembled to fix, but it's not critical to focus correctly. Horizontal misalignment is relatively easy to adjust: unscrew a short screw in front of the camera, right next to the viewfinder (to the bottom left corner). Behind it there's a tiny screw that regulates horizontal alignment. Just be gentle, it's a small screw on a small, moving part inside. Use no force. Adjust, check, adjust, check.
If the lever visible in the lens mount isn't as agile, as it should, and sticks in a given position or doesn't move at all, it may need some oil.
Check if diopter adjustment works. It should zoom-in zoom-out the image while looking through the viewfinder. I leave mine zoomed out, I wear something like -4d glasses, so I'm able to work this way. Despite what people here say, Zorki 3/4 is not glasses-friendly, there's not much of an eye point - Zorki 1/Zorki S type of viewfinder works better for me, I don't have to fiddle with my glasses. If you're worried with scratching glasses, use a single piece of electrical/duct tape on the top of the ring and you're set for a while. However, you won't be able to frame correctly with glasses on.
If there are no obvious flaws so far, put a test roll in the camera. Prepare a "test scene". With Jupiter lenses at f/8-f/11 you should be barely able to tell apart millimeter lines on a ruler from around 3m (close to image center). Set a tripod or put a camera on a table, measure the distance. Use moderately flat subject, like a bookshelf or a small table. Put something 30 cm closer, keep something slightly in the back. Use something with a small regular pattern, like a ruler or a printed page. Make two blank exposures, frame the subject carefully, draw on a piece of paper at least a postcard-size sketch of what's in frame and what's outside of it - this way you'll know how to frame these marvels next time. Diffused or flat (a lamp directly above the camera) lighting works better, but the test subject should consist of contrasty non-reflective objects.
Be sure to make one exposure at f/2 and one at f/11 and whatever you need in between. There's not much sense of shooting test images above f/11. Note the parameters of each shot. The first is to test the focus accuracy, the second - to test the maximum lens resolution, so you may repeat the same test with your best lens and compare the results. Be 200% sure the camera is stable and doesn't move between the shots too much, and that it's focused in the way, that makes you sure you know what you're doing: measure the distance with a tape measure, and check if the rangefinder shows ore or less the same. Note down the difference, when measuring both ways (I'd go with whatever rangefinder wants). Put something heavy on a camera if it would wobble. Shoot 1-2 more frames around the house. Change the lens and repeat. Take the camera outside. Make sure the camera got some sun from all six sides (this is how I check for light leaks, with a lens cap on). Shoot some more as you would shoot normally. Take notes. Shoot something at the fastest shutter speed. Be sure to shoot something totally even and featureless, best if at 1/60 - as said before: blue sky works great. The sun may burn a hole in the curtain, so use a cap when not shooting. Use a flash for some shots if you use flash at all. I don't.
Get back, develop the film ASAP. Check for signs of light leaks on the film. Check frame spacing and alignment. Check for even exposure across the frame on the sky photo - an uneven exposure with lighter and darker vertical band(s) is called banding. One reason is for these is disturbed curtain movement (=camera needs cleaning), the other - if eg. one edge is not exposed at all - is a sign of misaligned or unevenly tensioned curtains, it happens at the fastest shutter speeds mostly. One edge totally black is most likely a case of stalled grease in the curtain rolls.
Cut the film, scan it or make prints. Use a DSLR and enlarger if you don't want to waste paper - I'm puting DSLR body directly under the enlarger and use the sensor instead of a photo paper (focusing the enlarger is tricky). I'd only care for some specific parts of the image. First, I'd check if the focus is OK on the f/2 shot, or if BF/FF needs to be taken care of. Later, I'd go for the regular pattern in the center. I'd enlarge enough to see the grain really clearly and digitize this part of each test frame.
If you're lucky, you'll have two working marvels with two great Sonnar-like lenses. More likely both cameras will be off here and there and the lenses will differ in image quality. Nevermind what people write, Jupiter 8 is worse than, let's say, Pentax-M lens, but it may not be that far. It's a soft-ish, magical lens wide open and a good, sharp lens closed down.
Those are the things to look at when the cameras arrive. Quite a lot of things. This is how it looks in my case. A new camera is at least half of wasted roll. But if it works - it works, and I have my mind clear afterwards and I can focus on what to photograph and how, not how the tool will behave on a given date.
Those are old, crudely made, but reparable cameras, most likely serviced every decade or so. What went wrong with my four rangefinders: 100% had rangefinder misalignment, one stuck rangefinder cam (corroded Fed 5 body), one shutter with an old grease, all FSU lenses needed relubing, one (I-61 L/D) isn't sharp at all and has a fungus, so I think I'll learn something new with that buddy. All needed serious cleaning. All had good curtains.
Apart from that - curiously similar FSU SLRs are more prone to failures. I've got some broken ribbons and missing teeth somewhere under the wind lever. Also silver coming off of the prism.