You're wrong in two ways here. First, he had no idea what mutations are.
Really? Experts say he had some idea.
Nope. He only knew that variation was a fact. But the had no idea how it happened. Darwin even suspected that acquired traits might be passed on. Because he didn't know about genes, he had no idea about mutation. Ironically, Mendel had sent him a paper, but it seems he never opened it.
And of course, he was aware that variation is a random process, but also that, combined with natural selection, the result is not random. We observe that to be the case.
You have a logic problem above.
Nope. You can test this yourself. Roll dice, writing down a long list of results in order. But then any time you roll two consecutive odd numbers, roll again, replacing the previous result with the new result. Do maybe 200 of these. Then count the number of even numbers and odd numbers. If it's not close to 50-50, it's not random. A random process plus a non-random process, is a non-random process. This is why we see analogous organs and analogous organisms like bats and pterosaurs; the mutations were random, but natural selection is not random, so we end up with a non-random result.
Your logical error is to suppose that a single random element somehow rules out order. One of the things you learn in systems, is that this is a false belief.
Why do theistic evolutionists accept without a mechanical explanatory process that human beings come to exist?
Bad assumption there. In fact, we know of a number of such processes. One of the most compelling is the fact of neotony.
Human skulls look much more like the skulls of young chimpanzees than they look like the skulls of adult chimpanzees. Like young chimpanzees, our legs are relatively longer and arms relatively shorter than those of adult chimpanzees. Our crania are larger, our jaws and teeth smaller,and our faces flatter than those of adult chimpanzees. The foramen magnum (where the spine connects to the skull) is forward and under our skulls, whereas it is at the rear of chimpanzee skulls. Longer legs and the repositioned foramen are useful for a bipedal species. In an open savannah, with fewer trees as happened in the cooler and drier pleistocene, neotony was favored by natural selection. The shoulder joint in anatomically modern humans is radically different than that of other apes, and even of primitive humans. It permits a stronger and more accurate throw.
There is genetic evidence for these changes:
Nature 13 September 2023
A cross-species proteomic map reveals neoteny of human synapse development
and this...
The human brain stands out among mammals for its remarkably prolonged development. Synapses – critical connections between neurons of the cerebral cortex, the brain’s main hub for cognition – take years to mature in humans, compared to just months in species like macaques or mice. This extended development, also known as neoteny, is thought to be central to humans' advanced cognitive and learning abilities. On the other hand, it has been hypothesized that disruptions of brain neoteny could be linked to neurodevelopmental disorders such as intellectual disability and autism spectrum disorder.
The lab of Pierre Vanderhaeghen at the VIB-KU Leuven Center for Brain & Disease Research previously discovered that the prolonged development of the human cerebral cortex is mainly due to human-specific molecular mechanisms in neurons. Now, they are investigating these molecular timers in human neurons.
Human-specific genes regulate key gene mutated in autism spectrum disorders
erc.europa.eu
Humans are separated from their closest living relatives, the chimpanzees, by 6–7 million years of evolution. This is a short period in evolutionary terms: genetically, the two species are as much as 99% identical. Within this short time, however, human ancestors evolved a unique set of cognitive abilities distinguishing humans from other species. This raises the question: how, mechanistically, could human cognitive abilities evolve in such a short time interval? More than 30 years ago M.C. King and A. Wilson had already proposed that identifying differences in the timing of gene expression during brain development between humans and apes would be crucial for understanding human evolution. Indeed, change in timing and rate of ontogenetic changes, or heterochrony, has long been known as a potent mechanism of creating evolutionary novelties. If true, this mechanism offers a solution to the conundrum of human evolution, by allowing novel human cognitive abilities to develop on the basis of preexisting cognitive machinery. Comparison of human and chimpanzee ontogenetic changes on the molecular level, however, has visibly lagged behind those in model organisms. Here, we describe recent advances in this field, which imply a molecular link between the evolution of two seemingly independent human-specific features: cognitive abilities and longevity.
Lots more of that. Would you like to see some more?