No one ever says that Einstein came to his conclusions alone. It is like for any scientist, relying on the state-of-the-art of his time (the work of Lorentz and Poincare) and putting things together before going further.
Concerning de Pretto's work: Einstein derived E = mc2 from special relativity, which is not what de Pretto did. Relativity was not even discussed at that time.
Conclusion: there is no impostor or myth... Just jealousy maybe? :)
When you rely on something you have to give credit to those people of whom you will you base something he did not, other than that the only thing he did was plagiarizing the ideas of Poincare and Lorentz, these being better than in the field of relativity, if they were jealous because they won the nobel Einstein was not about "His supposed discovery of relativity" and if they recognized that Lorentz caregiver for more in 1902, three years before Einstein talks about it.
Many contributed, as in any science, and Einstein put things together to move forward. Of course he was aware of the works of his peers (although there is no proof to which degree). But he has not copied them and Lorentz and Poincare works have never formulated the theory of special relativity as such.
Einstein derived Lorentz transformation starting from special relativity. No one did that before. Lorentz transformations are called Lorentz transformations and not Einstein transformations (and similarly, there is something known as the Poincare group and not the Einstein group). This is where recognition is given to Lorentz to what he contributed for. Moreover, Einstein cited Poincare's work in one of his paper on relativity. Isn't this giving credits?
Finally, if you go back to the literature, you will notice that there are fundamental differences between Einstein work and Lorentz a d Poincare works. He is the one who got rid of the aether concept in the 1900s and the concept of absolute space. It is unfair from taking that out from Einstein (which is at the end why he his theory of special relativity is known).
I do not know what you need, but your claims are just not founded. And when I wrote about jealousy, I was thinking to people attacking Einstein today...