No, flexibility has nothing to do with the joints, it has to do with stretch reflex, while it is possible to elongate the joints a little, its strongly advised against as it weakens it.
You can achieve gymnastic levels of flexibility without stretching the joints, in fact you "already have", its your brain the one that tells the body not to overstretch, that's why you see the ragdoll effect on knocked out people.
Now, about glucosamine, Mayo clinic cites the evidence for knee health as strong and general arthritis as good, personally i used it and saw improvement, the problem with most people is that the studies that show improvements usually have doses that are 2 to 3 times the ones you see on packages.
Mine said 1 dose of 500 miligrams a day, yet studiest cited by mayo clinic talk about 1500 to 2000. Chondroitin also has very strong evidence to be good against arthritis.
Basically, take them at a decent dose and wear and tear pain simply seems to go away, i doubt its placebo effect.