Got the help of some connections and talked to a guy who was able to discuss things logically and scientifically.
And it matches with some of the advices above.
Fortunately I happened to have a 1.5T MRI at SKH not long after the physio, which showed nothing according to the report. When I asked should I pay SKH to give me the images the doctor said don't bother with paying the extra money, might as well go for a 3T MRI straight if there is a need to. Then he just showed some random examples he can simply find from google on the spot like this one
https://sportsortho.co.uk/mri-scan/the-value-of-3t-mri-scan/
Because there's also the issue of whether the guy interpreting the photos can do a good job or not. So might as well go 3T and get clearer images that even an inexperienced guy can see, rather than go 1.5T and pay for an expert to interpret and probably still do a worse job than getting a 3T scan to begin with.
Anyway, from the 1.5T MRI report, we can at least with some amount of confidence rule out cartilage or anything major. Putting aside the original issue of the left knee which probably has a more complicated root cause, the fact that the right knee was fine all along and only severely degraded after just one physio session and does not recover on its own after a month is sign of RSI due to poor form, since the symptoms also match that of RSI. Turns out what army fitness instructors tell you in English may not be 100% precise, and individuals may need slightly different form due to body differences. For example, during lunges, nobody should have toes pointing forward parallel, the optimal form should be slightly outward. And also while the knee should not go over the toeline, it should be moving naturally on its own stopping at around the middle of the foot, instead of being forced to stay over the heel. This is something the physio should have observed and corrected, but to be fair this is not easy to observe or judge.
Fortunately if it is RSI the damage is largely reversible, tho I was right to have stopped on my own after a week after testing and observing the results. The advice was that I stop doing squats and walking lunges - which were what the physio suggested - which shouldn't be done for bad knee joints anyway. Instead focus more on functional exercises - But wait, aren't squats and walking lunges functional, based on standard google results which is based on standard medical textbook? - Answer is not really, because they are not natural movements, which is exactly how people can injure themselves due to bad form, despite spending years or decades living normally without doing these exercises. Instead, just continue doing what you would normally would have been able to do in your daily life anyway, like climbing up or down stairs, and doing squats but with a chair. Or squat with balancing ball on the wall. This is to keep the muscles strong and also to relieve the pain, which at this point is likely due to muscle imbalance causing incorrect joint motion, which was caused by incorrect exercises to begin with.
Or in other words, even tho it has been feeling painful going up/down stairs for a month, don't run away from it, deal with it like pain is growth. Note that this is only because we confirmed that this is due to muscle injury and no other skeletal damage based on joint movement and pain behavior. For bad ligaments or cartilage, no, don't, pain is bad.
And true enough, after just a few days (or just 1 day? or 2?) of slowly going up and down the stairs, going up the stairs is no longer painful, and going down the stairs only has a bit of pain at a lot of flexion (which is unnatural anyway; we can just bounce down normally). Standing and walking have no issues too. Sitting down still has a bit of pain and soreness but the location has shifted, which indicates the exercises are working, and proves that the original exercises are the problem.
Also to experiment a bit, just to do a bit of the suspected offending exercise and see if symptoms flare up again. Right now definitely squats are flaring up my situation (didn't test lunges). Full recovery until able to squat without problems may take months, or may not be 100%. Or just accept that this old body just can't parallel squat 20 times anymore. What's the point of exercising to relieve pain when the exercise causes more pain.
After all that long talk, it feels like doctors are just technicians of the human body... just follow predetermined question and answers, no wonder their syllabus is dead memorize.
