I'm very sorry to read about this, we've been there/done that and there's no easy work around. Based on what You've posted I'm pretty confident that it's a torn ACL, the 'popping' noises are the torn meniscus (cartilage) rubbing each other the wrong way. I'm not playing 'doctor' or trying to act like a know-it-all, just based on my experience that's pretty much the identical symptoms that my dog had and it's all too common.
When my dog's first ACL tore I was worried that the extra stress on the good knee might cause it to tear as well so I got her in for a TPLO (just to get a solid leg under her). Turns out it was a good thing I did because the other one cut loose about a year later.
The thing that drove Me to surgery this last time (soft tissue repair) was the noisy joint, every time the meniscus 'pops' or 'drawers' it's doing irreversible damage. They wound up removing the whole meniscus on one condyle (medial I think) of one side of my dogs leg this time because it was so badly torn up, that pretty much guarantees that she'll have arthritic pain in that leg all too soon. If I would have known that they were going to remove the whole cartilage anyway I probably would have given conservative management more time, but this surgery wasn't the worst thing in the world. She's still working out the kinks but we're off restrictions and her leg is painful but solid.
Just FYI, to provide you some info to get started here's a link to a very pro-conservative management page:
Here's another link that has info on surgical fixes: soft tissue repair, TPLO and TTO:
This last time I tried conservative management for about a month, that's when her leg started popping, she wasn't showing improvement and I just couldn't deal with the sick in the gut feeling I got every time her knee made that noise (every....friggin.....step, pop, pop, pop). The vet surgeon said Her cartilage had fully folded back and she was walking on the underside of the meniscus, ouch.
When I talk about meniscus and cartilage (same thing) think of the white stuff on the end of a chicken bone. That's an 'articular' surface, it's like nature's teflon. It provides a gliding surface for the joint, nice and slick and smooth. When it wears out or breaks there's bone-on-bone contact. Bone regrows, cartilage does not. Irritation of the bone (through constant rubbing against bone) causes scar tissue, regrowth, osteophytes and abnormality that wouldn't occur in a normal (even with thin condylar tissue) joint.
Please bear in mind, every 'pop' You hear is heavy wear and tear that won't heal. If You can keep the joint from popping it may settle back and heal, I dunno, I just couldn't deal. Like I said, they took the whole thing out because it was wasted.
No stairs and movement restrictions bordering on abuse are required no matter which route you take, it'll break your heart sometimes but setbacks just prolong the restrictions.
Kimmy's mother had a torn ACL when she was whelping the pups, I had some foreknowledge that this might be a problem and I totally agree with You that it's a genetic issue. Both parents had such wonderful temperment and conformation that I just said, "gimme a puppy" and I never regretted my decision. The breeding program at that kennel got kind of whacky after Kimmy's litter but her pedigree was pretty solid. Her hips and eyes are good, but maybe weak ACL's need to be noted in AKC standards? Like, dogs that have had knee surgery shouldn't be bred or shown? This is all too hella common lately.
I hope the links will be informative for You, if you have any questions please ask.
Best wishes to You and Loki.