whatever distress the individual presents during therapy it is regarded as an issue of living rather than a mental disorder per se

Nice! Blame the sin, not the sinner, so to speak.

While its main idea may be simple, trying to put them in practice is not that straight-forward as humans seem to be inherently judgemental.

Quite so. Every conscious thing is of necessity judgemental, because every conscious thing of necessity has values, so it will look at things necessarily through the perspective of these values. A therapist might try, and partly succeed, in being non-judgemental, but I don't they can ever truly deeply succeed. I think that's more a job for, say, one's partner: after many years knowing us, they should become experts on us!

every human being is a singular being

As unique as a grain of sand. In other words, strictly, but irrelevantly, unique! Meaning: grains of sand are more similar than they are different. I think uniqueness, like most things in life, must be worked hard at. It's not something gained by default. To paraphrase George Orwell's Animal Farm, "all animals are unique, but some animals are more unique than others"! Nietzsche saw amor fati as a thing attained only after much struggle. You basically have to be a genius to do that; only very rare people can do it (according to him). And in my interpretation it has to do with being a Creator, being able to use negative things to your advantage, like David Bowie used his odd-colored eye that he got in a fight at school when someone supposedly stubbed him in his eye. He used it as a trademark of his appearance and his art and became known for it and was imitated later by Marilyn Manson with his oddly-matched contacts. It's a display of extraordinary will ("will to power"), basically, to use the negative to one's advantage.

Anyway, nice write-up, glad to learn a few things about existential psychology, I don't think I was aware of this beforehand (unless something escapes my memory).