And please remember: before "seeing" electrons or photons, our friends in physics could make all sorts of predictions, and many of them turned out to be true.
But, ultimately you are correct: much a priori assumptions.
“evidence does exist for, say, chimp-human shared ancestor in the closeness of the DNA compared with gorilla or orangutan-human DNA.”
Yes, there are similarities, and that can be considered evidence, but ultimately, if it were a court of law, it would be circumstantial evidence. Logically, similarities can arise in different ways, even evolutionists admit this with their theory of convergent evolution.
I think there would be much less conflict between the scientists and the skeptical public if they would admit that certain aspects of evolutionary theory are a very speculative science, rather than trying to paint the entire field as true practical science. Parts of it are practical, but I think tying them together with the speculative parts is a just a result of defensiveness.
Take, for example, higher physics. Some of it is very practical, and well established, and some is purely speculative at this point, like String Theory. The physicists make a clear distinction, and nobody really cares if they make outlandish and possibly wrong postulations in the speculative areas, even if those ideas, were they proven, might shake up someone’s worldview.