Looks like you need a quick lesson in "Hinduism".
Hinduism, the word, is a misnomer. The official term is Sanatana Dharma (Timeless Order). Thus, to say that "Yoga", which happens to be a fundamental school of philosophy ("Darshana") of Hinduism, is not only absurd, but obtuse. This is a ploy by anti-Hindus and anti-Indians to claim that the philosophical insights of Hinduism actually "predate" it and was practiced in some other context.
Sanatana Dharma, simply put, is the human experience of the divine. The philosophies that arose from the banks of the Saraswati River (now in pakistan) all the way to the shores of Kanyakumari (southern tip of India) are collectively lumped together as Hinduism.
There are six main schools of philosophy in Hinduism:
1.) Samkhya - Physical/Metaphysical Interpretation of the Vedas
2.) Nyaya - Logical Analysis
3.) Vaiseshika - Atomic Theory
4.) Purva Mimamsa - Scriptural Exegesis (Ritualistic Interpretation of the Vedas)
5.) Uttara Mimamsa (also called Vedanta) - Philosophical Exegesis (Philosophical Interpretation of the Vedas)
6.) Yoga - Transcendence and Spiritual Practice (Sadhana)
The last two, that is, Vedanta and Yoga, are the predominant schools of philosophy in practice today.
These are the "astika darshanas" that uphold the Vedas.
There are three nastika darshanas that deny the Vedas but still are considered part of Sanatana Dharma:
1.) Jainism -- extreme non-violence
2.) Buddhism -- non-theistic
3.) Carvaka -- athiestic
There is plenty of evidence, as you've stated, of archeological artifacts that depict figurines in yogic postures. This is ample evidence, as suggested by real Indologists, not foreigners with a christian agenda, that yoga has been practiced by Indians for thousands of years. This means that the philosophy behind Yoga, which is completely Hindu, was practiced then as well.
The Saraswati River basin has been photographed by satellites and proven that it dried up before 4000 BCE. The Rig Veda, the oldest extant texts of mankind and a fundamental part of Hinduism, revolves around the Saraswati River. Thus, it is amply evident that Hinduism was prevalent for far longer than so-called historians and "Indologists" who have an anti-Hindu agenda, state. In fact, the first Indologists, primarily from England (William Jones) and Germany (Max Muller), were christian missionaries out to show Hinduism and Indian civilization in a poor light. Once they realized the civilization was far superior to anything they had ever come across, they came with absurd theories and began to spread them through the universities back in Europe as sacrosanct. One glaring example: the now-debunked Aryan Invasion Theory.
There is also linguistic evidence, primarily from the Rig Veda, that the composition, even according to anti-Hindu "scholars", occurred by 1700 BCE. This is when things were "written" down. The funny thing is, in Hindu tradition,
Shruti, or "that which is heard" is given more importance to
Smriti, "that which is remembered". The tradition of passing down knowledge from one generation to the next orally was prevalent for thousands of years PRIOR to writing things down. Thus, the actual date of the Rig Veda, is severely contested and there are plenty of opposing views. Some say, due to the archeological evidence of the Saraswati River basin, the Rig Veda was in use 6000 BCE. Some say 4000 BCE. And others, 1700 BCE.
The 1700BCE is a farce for a couple of reasons. The European Indologists had a clear agenda of denigrating Indian/Hindu civilization and so rewrote the history of India with their bias; this is unfortunately what is taught to even this day in schools and universities around the world despite the glaring evidence to the contrary. Plus, since they were christian missionaries, they idiotically believed the earth is only 6000 years old! So to say that something or a civilization is older than that would have been absurd. They then redacted the dates and came up with this much more recent date of 1700 BCE to corroborate their imaginary "Aryan Invasion Theory".
This has been thoroughly debunked but due to the gross Eurocentric viewpoints of most academicians in India and outside, the evidence is not presented to the students and some people are still making a living holding on to that hoary theory.
Getting back to Yoga, to say that it predates Hinduism is not only completely erroneous, it is just redonkulous. The Bhagavad Gita, where the Mahabharat war at Kurukshetra took place around 3139 BCE (again, lot of discrepancy in dating the war), Sri Krishna talks about the four Yogas specifically. To have such detail of Yoga described in the Gita means that the civilization was already well aware of it at the time. The only religion, or rather system of philosophy, prevalent at the time was Hinduism (Sanatana Dharma).
If you were to go even further back, the Ramayana is actually called "Yoga Shastra" or Yoga Treatise. The philosophy of Yoga is imbued in Indian culture and Hinduism so deeply that it is a product of the advanced spiritual science that has been conducted in India for thousands of years by seers, sages, and saints in an unbroken lineage.
Regarding the four Yogas:
1.) Raja Yoga -- Yoga of the mind (manas) -> intense concentration/meditation
2.) Jnana Yoga -- Yoga of knowledge -> rational inquiry
3.) Karma Yoga -- Yoga of action -> righteous action
4.) Bhakti Yoga -- Yoga of devotion -> selfless devotion/submission to the Supreme
If these Yogic philosophies, and each one is very deep, are so detailed and prevalent in Hinduism, if what you say (that Yoga predates Hinduism), how come they were not used in other "religions"? Aspects of it are, but the core philosophies of Yoga is not!
Hinduism is such a misleading term because the westerners who wanted to know what religion most Indians practiced, they just redacted all the philosophies to fit their limited understanding. Take the glaring differences between Hinduism and the other religions:
1.) Hinduism has no "founder"; it is Timeless Order or 'the way things are' (i.e. describes the nature of reality)
2.) Hinduism has no "book" or one set of rules to follow; it reflects the culmination of the spiritual experiences of the sages of India over thousands of years
3.) Hinduism is the only religion that states there are many paths to the divine (from the Rig Veda itself) --even Buddhism doesn't state this
4.) Hinduism has no founding "date" or period. This is a concoction by western Indologists who cannot bear the thought of a philosophical system far superior to anything they had ever come up with
Regarding what you say about Hinduism being like an amoeba, the cultural aspect of Hinduism may be this way but certainly not the philosophical aspects.
The principles of reincarnation, karma, meditation, realization etc. are fundamentally "Hindu" or part of Sanatana Dharma. Those who state otherwise are simply disingenuous.
And, if I were "conditioned by my Hindu upbringing", I'd say that Hindu civilization is millions of years old. The Vedas are known as "apaurusheya" or "not of human origin" and are thus eternal. They are revealed and discovered repeatedly in different cycles of civilization by different sages and seers. I would also say that Sri Rama lived about 1.7 million years ago and fought a cataclysmic war with Ravana of Lanka and vanquished him. I don't say that because there isn't ample evidence to support this; at least not yet. Well, actually, NASA did find the bridge from southern India to Sri Lanka that has been dated to 1.7 million years and is exactly the way it is described in the Ramayana! I'm still holding out for some more evidence and then we shall see.
Remember, ancient India included all of Afghanistan, eastern Iran, all of Pakistan, Bangladesh, Nepal etc. Apart from the Indus Valley Civilization, which by the way was undoubtedly Hindu (they found Shiva Lingam in several disparate places that date more than 3300 BCE), there hasn't been much archeological digging. Time will tell.
Last but not least, to say Dhyana started in China is laughable. Yoga, of which dhyana is a key component was taught to the chinese by Bodhi Dharma. Since the chinese couldn't concentrate their bodies enough, he taught them martial arts which later on became known as Kung Fu! And meditation has a certain purpose; that purpose is only described in Hinduism and now Buddhism/Jainism. Prior to Buddhism/Jainism, both of which are offshoots of Hinduism, only Hinduism (Sanatana Dharma) existed.
And, to be fair, I am completely open to true archeological evidence and if there was some irrefutable evidence that was discovered that Krishna didn't exist, it has no effect on me or any other Hindu. That is the greatness of Hinduism; it doesn't depend on one person, or one event, or one location; it is Sanatana Dharma and thus describes the nature of reality. That is why the motto of India is SATYAMEVA JAYATE (TRUTH WILL TRIUMPH). Hindus uphold truth, no matter how uncomfortable it may be.
I wonder if christians are reasonable enough to look at the evidence that jesus traveled to India to study with the Hindus/Buddhists there and learned Yoga? There is ample evidence for this but bring it up to any christian, and see what they say!
Unfortunately, all other religions depend on historical events and people too much to stand on their own.
If you do some research on the latest evidence of the Indus Valley Civilization and Hindu philosophy, you will clearly see that this great civilization arose in India and went from there to the rest of the world.