We have enough experience with the Clintons and this administration to know not to take denials at face value. You might recall explanations that Benghazi was a demonstration about a video and the IRS had done nothing improper.
You are studiously ignoring the main issue, which is how extremely confidential documents found themselves on her server. I find it highly implausible that SAP, ie above Top Secret, material, was being sent around the State Department without any classification, particularly since it would most likely have originated outside the State Department, eg CIA. Someone had to make an intentional effort to move that material from the secure system to her unsecure system. My understanding is that such transfer is criminal. How that happened should be under investigation by a grand jury. Your assumption that this transfer was a mere "screwup" is unwarranted, but even if true, it shows the utter disregard Hillary had for security and rules. If she hadn't been using a private server, there would have been no breach.
This security breach is not an isolated incident. We know there was a troubling pattern of massive donations to the Clinton foundation that seemed to pose conflicts with her official duties. Proving a quid pro quo may be difficult but the appearance alone is sufficiently damning that it must be investigated fully and openly. At least that was the standard applied to republicans.