While I'm skeptical of these claims of bodily healing through psychedelics - of course your experience with tackling Post-COVID syndrome are likely much different - I'm very in favour of believing it. Gives the overall experience a great narrative.
I am quite certain that serotonin pathways initiate tissue remodeling and/or regeneration, which is kind of a low level form of healing. This is well established science, albeit not widely known because much of the relevant work is relatively new. Most people understand serotonin to be a neurotransmitter. In reality serotonin is a hormone which is produced and/or utilized by practically everything under the sun. Indeed, it may have originated
because of the sun and the energy capturing properties of indole rings, particularly in the amino acid tryptophan. Tryptophan captures blue light energy which can be used to catalyze interesting chemistry but also leads to collateral damage from free radicals. At one time, these process may have formed some serotonin (among other things) spontaneously. Some time later, organisms may have evolved to treat this serotonin as a signal for the need to control damage and initiate repair processes. Later still, organisms may have evolved ways of purposefully synthesizing and storing serotonin, which allowed the damage-control-and-repair machinery to be activated under different circumstances---such as in response to other kinds of damage. Indeed, the release of large amounts of serotonin in the region of a wound is observed in some plants and animals. In humans, most serotonin in the body is found in platelets where it can be rapidly released in the blood vessels at the site of a wound.
So essentially yes, serotonin induces healing at a kind of low level, but whether this action therapeutically beneficial is very likely circumstantial. Chronic excess of serotonin in the blood stream causes significant health problems including the valvulopathy that 5-HT2B agonists are known for. Having said that, serotonin is but one ligand that is active at serotonin receptors, which are named "serotonin receptors" because serotonin is the first known ligand for them. We know LSD and other psychedelics exert a type of action at 5-HT2A that many other 5-HT2A agonists and antagonists do not exert. In other words, the 5-HT2A receptor is multi-functional, and so while psychedelics do mimic serotonin to an extent, some mimic it poorly while strongly activating the receptor in a qualitatively different way. Thus far, almost all study of LSD has been in the brain and not anywhere in the rest of the body, yet I strongly suspect that the body produces ligands with LSD-like action even if not necessarily in the brain. (I'm skeptical that DMT/5-MeO-DMT are relevant.) I have a hunch that if we studied the action of psychedelics in our cells we might discover some potentially powerful medicines and get better insight into what they are doing in our brains.
Very glad after all that I took the 2C-T-2 yesterday. I'm certain it will not disappoint in higher dosages. The trip was impressive given the little amount that I used, had a nice plot too. As you described, initiated a healing process and after wading through the mud, salvation awaited. I like a good psychedelic with some catharsis and the duration is right up my alley too.
I'm glad you feel it left you in a better place. A few people find the body load to be too much to make it worth it, but a lot of us do it despite a rough come-up because the rest of the trip is so nice.
I'm not sure if you read Myron Stolaroff's "From Thanatos to Eros", but here is a relevant excerpt for you on the topic of bodily discomfort with 2C-B:
Why yes, I have read his book. The negative karma concept is very interesting. I believe it is borrowed from somatic psychology theory, which is also very interesting though not widely practiced or accepted in the mainstream these days. I increasingly lean toward the view that much which is considered of the domain of mental health or psychology is actually misunderstood physiological phenomena. Essentially if the patient is complaining and the doctor runs a bunch of diagnostic tests but doesn't see anything wrong, then it is presumed to be a mental health problem, and the patient is referred accordingly. The trouble is, the doctor may have not run the right test to properly diagnose the disease, or perhaps the right test doesn't even exist.
Meanwhile, I'm really coming to the view (in light of the actual science described above) that psychedelics are widely misunderstood to be primarily or exclusively active within the brain. The brain activity is absolutely important, but trips really are a full body thing. So if I may make a suggestion. Perhaps this concept of negative karma can be better understood as a kind of maintenance debt. It's not so much about "toxins in the muscles" as simply damage for which repairs may have been deferred for one reason for another. We can all understand why the body might opt not to repair some tissue in so far as it requires resources that may not be available and may temporarily reduce the performance of the tissue while it is being repaired. Serotonin and/or psychedelics may signal to those tissues that it's time to fulfill the debt, to carry out deferred healing and repairs. Something psychedelics may do uniquely is alter the process by which such healing and repairs are carried out in a way that may effectively short-circuit the usual serotonin-activated process and which might, in some cases (like say chronic inflammation), be more therapeutically beneficial. Perhaps "negative karma" is accounted for by up-regulating and down-regulating serotonin receptors.
Against your theory of physiological healing there is the fact that some psychedelics are virtually free of bodyload (this is largely true of 2C-B, 2C-C and 2C-D for me). But perhaps they are not as effective on the body - good medicine has to be bitter!
It's not against my theory at all in so far as I admit that 5-HT receptors are multi-functional. At the very least we know that some 5-HT2A agonists are LSD-like and cause tripping and others do not cause tripping. This is one possible way to explain why psychedelics differ so much with regard to "body load". My understanding is that LSD actually has very little serotonin-like effect at all. Some researchers describe it as an antagonist even, but in terms of psychedelic action, it is very much an agonist.
Could it be that serotonin-like action causes body load and LSD-like action does not? Data I have seen does suggest that drugs like 2C-B, 2C-C, and 2C-D have lower serotonin-like "efficacy" compared with drugs like 2C-E, 2C-T-2, and mescaline. Unfortunately, almost all these things also have action at other 5-HT2 receptors and also at 5-HT1A. Activating the 5-HT1A receptor can sometimes have opposite effects to activating 5-HT2A. Activating 5-HT2B In the brain (and maybe other places like nerves?) enhances overall serotonin transmission, meaning more serotonin is competing with the psychedelic in the 5-HT2A receptor but also more serotonin is activating 5-HT1A too. If 5-HT2B enhances serotonin transmission in nerves in general, then this could have interesting consequences. The brain can cause nerves in particular areas of the body to secrete serotonin, not merely for transmission but to potentially alter their metabolism. In this way, both entactogens and psychedelics with 5-HT2B activity may greatly enhance the ability of the subject to consciously direct healing to specific parts of the body.
I admit that much of what I'm saying is speculative, but it cannot be taken for granted that serotonin pathways are expressed all over the body, and psychedelics undoubtedly have interesting and profound effects in more places than just the brain. If we knew better what these things actually did, it really could revolutionize medicine.
:D 
just kidding, I hope all is well Kaleida
Those equations were a long time in the making, I've been tweaking that system for close to a decade now.
I'm more than happy to discuss it here as well.
I hope you're doing alright as well. 
