Thanks for using the Forum!
I understand your concern, and remember during the Yin Yoga course we talked about creep. It can and does happen: during a long held stress, fascia will creep and thus lose some strength. This is why all Yin Yoga classes use counterposes and shavasana--to give time for the tissues to recover and to speed up that recovery. As I mentioned in the course, I would not recommend someone do Yin Yoga just before sports. If you want a refresher on this topic, you can read my article on Creep and Counterposes
Your concern over "high-fiving" someone after Anahatasana could be valid if - there was no counterpose, and there was no time between doing that posture and high-fiving. In reality, this would not happen. There is time after the pose, during shavasana and leaving the studio which should be sufficient to allow the creep to resolve. Theoretically, anyway.
The important question is - what do you feel? Does your shoulders feel fragile after class? Have you ever experienced a problem?
I do not agree that "the joint needs to be strengthened rather quickly", it just needs time to recover. I would not suggest doing handstands right away or lifting weights for at least 30 minutes, but simply daily movements should not be an issue. But, that will vary for every student.
Regarding the fibroblasts, I am not sure what your chiropractor means when he says that they will do what they do and will just open. They don't behave like that. They don't "open". For more on what they do, I would suggest reading the work by Helen Langevin which I discuss in this Forum Thread
. There are specific cellular responses that the fibroblasts make when subjected to long held stresses.
And, then there are all the other physiological responses we have to these stresses, such as the change in the state of water (from gel to sol). These are discussed a bit more in my article A Scientific Basis for Yin Yoga
You also ask about strengthening: yes - we do need to strengthen the joints too, but that happens by stressing the joint. Strengthening the muscles around the joint will actually take load of it. That is great if the joint is weak, but better is to strengthen the yin tissues of the joint. Remember the quote in the course from Professor Laurence Dahners, “A common clinical finding is that unloaded ligaments not only atrophy, but also undergo contracture., Unstressed fibroblasts increase and contract their actin skeletons causing contracture. An absence of stress generated electrical potentials (SGEP or piezoelectricity) increases contracture. We need to stress these tissue, not just strengthen the muscles. Stressing these tissues forces them to adapt and become thicker, stronger and more resilient.
Again, all of this is theoretical. What is important is your experience. Do you feel fragile after Yin Yoga so much that you are fearful of giving someone a high-five? If so, do more counterposes or take more time before subjecting the joint to large stresses.