Boogieman:
"Well, here’s one way it works: they find some primate bone fragments, maybe a few teeth, half a jawbone and a crushed femur." In fact, there are dozens of different pre-human "homo" & other species fossils, from Ardipithecus ramidus through to Neanderthals, Denisovans and Floresiensis "hobbits".
Some of these, as you say, do indeed have only one or two samples so far found.
But others (i.e., Neanderthals, Ergaster) have dozens or hundreds of individuals found.
These allow reconstruction of complete skeletons, and from them presumed outward appearances.
Point is: there's more than just guess-work behind those reconstructions.
Boogieman: "Some scientists come up with a conceptual idea of what that primate must have looked like, based on their preexisting bias that the primate must be an intermediate form between other primates and homos."
The bones themselves tell a lot about what those individuals looked like -- no need to exaggerate either their human or non-human aspects.
An honest representation, to the best of our knowledge, is all that can be expected.
Boogieman: "Then they show you a sequence of such conceptual renderings and call it 'objective'."
Hominid skulls, from chimpanzee to modern homo sapiens:
“Some of these, as you say, do indeed have only one or two samples so far found.
But others (i.e., Neanderthals, Ergaster) have dozens or hundreds of individuals found.
These allow reconstruction of complete skeletons, and from them presumed outward appearances.”
Sure, for a few examples, this is true. For the majority, the fossils are laughably incomplete, and the reconstructions are therefore of dubious quality. There are certainly plenty of examples of evolutionists letting their imagination run wild over a few bones, declaring a new human ancestor, only to find a few more bones which showed they had merely found a new kind of ape.
“The bones themselves tell a lot about what those individuals looked like — no need to exaggerate either their human or non-human aspects.”
Only for the few examples where we have mostly intact specimens, which is not many.
“Hominid skulls, from chimpanzee to modern homo sapiens:”
Which proves nothing more than that scientists are able, much like kindergartners, to arrange various items according to some property, such as size or general similarity. This might impress schoolchildren, but it is of no actual scientific value.