OP, the upper-level engineers didn't have access to the code until AFTER the acquisition was completed and had zero knowledge of it until it was announced. This was not how previous acquisitions had worked.
You have to understand, at the time there wasn't even a CTO in place and there was no one in Engineering who had a seat at the table at the time. The lack of due diligence of Scamfecta has nothing to do with Engineering.
For some perspective, there are a lot of things that happen at the VP level out of the blue, and everyone under them, including Sr Dir, Dir, managers, etc, have to clean up and yet have zero input or involvement in.
Point your rage to that level; anyone below that is just dealing with the fallout of the whim of VPs. As long as they are competent and not like narcissistic future VP types, they are trying to do their best for the people below them.