Remember that culture is always a moving target. Let's take that description...
I audited a class on Native American sacred traditions once; a lesson I recall was on the nature of shamanism. This might have very little to do with Thule shamanism, but the basic syndrome is:
Someone gets sick, very sick, or have some other severe life crisis afflicting them. They go into a dream state, and encounter some non-human Power or other, that as the teacher (himself a Cahuilla Indian) described it "makes you an offer you can't refuse." When the victim recovers, he or she has new insights and powers--but is not revered, or followed, the way a secular leader is. They are uncanny and people tend to keep their distance, because no one quite trusts their power. But they come in handy so they aren't outcasts--just viewed with caution and suspicion.
What you've described among the Thule as "shamanism" seems like something else to me.
Actually, it's quite like Thule Shamanism in its early history. It's a good picture of Shamanism for a hunter/gatherer society wrestling with the spirit world. Thule Shamanism circa 800 or 1000. Not the Thule Shamanism circa 1500 or 1600.
But its worth talking about the Shamanism of 800 or 1000, to try and begin to illustrate how it changes over 500 or 800 years.
I think I've confused people with my relentlessly pragmatic and dispassionate approach to things. Certainly devout Christians will feel uncomfortable around me. And in writing about Shamans, I'm often shocked when someone reads this as me describing or inventing a proto-scientist genius class... when I felt that what I'd talked about was superstition driven trial and error machines.
I've written over and over again about the Shamanic tradition as a way of coping with the fundamental uncertainties of a subsistence existence, particularly a hunter/gatherer existence, and the use of 'magic' to try and manipulate the overarching randomness - to guarantee or affect everything from good hunts to illness.
Okay, let's look at it from another point of view. From the subjective experience of the Shaman... which is really where your quote comes in.
I'll make some comments about your quote though. To the extent that these things are or can be chosen, they're frequently chosen.
Sometimes they fall into it 'ie, are chosen by the spirts or circumstance' That's the supernatural 'offer you can't refuse' - its rarer than is let on. It happens sometimes, but most people in one sense or other go looking for it. But there's a cachet to say or believe that you were summoned - a lot of religious leaders talk about the 'calling of the lord', and there's also in modern parlance the talk of 'callings.' Political leaders all like to claim that they were chosen or drafted or enlisted for the job that they fought and clawed and pursued.
But in almost every case, the reality is that the person who was 'summoned' or 'called' was out there looking for it, or at least making themselves very open to the experience.
The people who end up being Shamans tend to be social outliers, they're smarter, less socially competent, there may be syndromes that in our culture are described as mental illness - bipolar, manic, schizophrenic, prone to hallucination, or other syndromes. They are often intense.
Persons showing such features are often selected by the community for shamanism, or select themselves, once in a while they fall into it. This selection can come early in life. It can also happen much later in life. I suspect that if you were to collect a statistical sampling of case histories for Shamans, you'd find clusters around puberty through early adulthood, a smattering of cases through the productive adult years, and then another cluster in senior years.
The Shamanic existence is often accompanied by trauma or extremity. Sometimes the trauma is right up front, catapulting a formerly fairly ordinary person into a mediator for the supernatural. That's a good line to claim by the way. Mostly, the traumatic experience or extremity comes after a lot of training and preparation, learning and apprenticeship.
There's a lot of literature the world over of Shamans undergoing or entering altered states of consciousness either through psychedelic drugs, intoxication, fasting or famine, or torture either self torture or outside inflicted.
In addition to mystics of every faith, S&M fanatics describe a Euphoric state of consciousness called 'subspace', marathon runners describe a 'runners high'. Extreme monomania - ie, forced repetitive chanting, or meditation can do the trick. These are not usually accompanied by hallucination, but there's a strong ecstatic/euphoic component.
Extreme illness can produce the altered state of consciousness, I once had a fever so bad I began to hallucinate strongly. I was lucid through the hallucinations, but accepted their reality without question. I can still remember the vividness. Extreme emotional crises can produce many of the same symptoms.
This is the dream state. This is all the shamanic dream state is. It's violently altered brain chemistry, a sense of surreality, super-reality, visual and auditory hallucination, and hyper-cognition. There's all sorts of ways to get there.
Different cultures will characterize it in different ways. But the core is the same, regardless of whether its a sub being whipped, a mystic starving himself, a buddhist meditating, a hippie on hallucinogens, a runner on a marathon, etc. etc.
Now, where culture makes the difference is how the individual contextualizes or receives the experience. When I hallucinated, it was about floating machines and floating engine parts. A Christian will feel Christ close to him. A submissive is self involved. An animist will commune with spirits.
Doing that to yourself is risky. Schizophrenics often feel impelled or controlled by external forces. Get into an altered state of consciousness where your messed up brain chemistry starts creating auditory and visual hallucinations, you will find that those hallucinations are very very insistent on being listened to and obeyed.
Shamans are regarded with a bit of wariness for that reason. Altered brain chemistry is a screwy dangerous thing. Remember Abraham being told by god to sacrifice his son Isaac? Or the story of Hercules afflicted by God-sent madness and slaughtering his family. People in that state are potentially dangerous, and even when the danger is past, they never quite stop being unpredictable and dangerous.
But there's a value. Altered states of consciousness are also associated with unusual creativity or insight. Artists or musicians often refer to a 'trance'-like state when involved in intense creative activities. There's the famous story of the shape of the DNA molecule coming out of a dream. Intuition, hunches, the flashes of insight... That can be useful to a community. It can also be useful to the practitioner within a community.
And there's the cachet. People are no stranger to suffering and discomfort. Someone who undergoes the kind of serious trauma, fasting, flagellation or whatever, that induces brain chemistry screwups - you respect that kind of stuff. You know what pain feels like, you know what hunger feels like, you don't like that stuff. Suddenly, you meet someone who not only does not run away from that crap, but they go up and ask for seconds? Respect.
Of course, genuinely crazy folk are not going to live too long. The schizophrenic in a primitive culture who is so seriously screwed up that he can't tell when he's on fire... he dies, either natural causes, accidents, self-inflicted illness, starvation or his peers kill him.
This is the other side of Shamanism, the deliberate humbuggery. By this, I don't mean fraud. There may be some of that. But the reality is that you can't spend all your time in altered consciousness, you have to come down from that. And you have to deal with people, which includes dealing with people who want things from you.... answers, cures, intersession with the spirits, etc.
So, you're an ordinary rational person... you're smarter than average, you've had a lot of experience or history with people coming to you for healing... what are you going to do? You make like a Doctor, you pick up some genuine healing - potions, plants, medicinal techniques, either by trial and error, or from other Shamans - you learn what seems to work, and you go with that.
You also learn to read people really well. Comes with the territory. You're in a people profession - lawyers, doctors, poker players, policemen, con-men, they learn to read people, they learn to pay attention, to look for things and respond to that. A good Shaman is an astute reader of people, and manipulates or uses those readings.
If its part of your culture, you learn the dances, you learn the chants and songs, or you make up your own - partly mystical inspiration, but partly rational calculation. You take credit when things work, you blame the spirits when things don't work, and the way people are, they remember your successes.... unless you've had a big string of failures or a critical humiliation.
This is your Thule Shaman from 800 CE, and perhaps your Shaman from most every culture with a Shamanic tradition. You have a caste of people who don't fit in, who are generally smart and talented, and they're diverted into a lifestyle where they induce a profound altered brain chemistry often as a profound shaping moment, and thereafter from time to time, and outside of those altered brain chemistry situations, they're smart people readers, smart trial and error machines, social arbiters, thinkers, etc.
Their viewpoints and outlooks are profoundly mystical, you can't avoid that. They're having flipping hallucinations where spirits come and talk to them and insist on them doing stuff, where they believe that they turn into and out of animals, etc. This is subjectively real to them, its part of their experience and world view.
But they're also within that framework a rational, canny, effective and manipulative group... because they have to be. Because the really crazy ones, the really far gone mystical ones... they die.
This was the subjective world of the Shamans of 800 CE among the Thule, and to some extent, it's still the subjective world of Shamans of 1500.
Did Grandfather fast or self flagellate or suck back a shitload of magic mushrooms and have conversations with walrus and unicorns and transform into a caribou and gambole with spirits. Sure he did. He believed every bit of it. But he didn't do that all the time. He made it work for him. And when he wasn't high as a kite, he was a thorough schemer.
Manupataq, on the coast of Greenland, huddling between two Caribou, half starved, shivering, her body temperature slowly dropping towards hypothermia, half conscious, her memories full of villages of the dead, rotting whales on the beach, dogs eating corpses in the streets, stories of survivors ringing through her head.... the realization that Christ is the spirit bringer of plagues and sickness, that's as much a mystical revelation as a logical conclusion.
Sometimes mystical revelations plays us false. Sometimes it worked. For a large part of our history, prior to the scientific method, mystical revelation was pretty darned effective in terms of motivating people, getting stuff done, and even producing some degree of innovation and cultural and technological advancement.