There's a simple reason why the FBI accepted the CrowdStrike finding and never insisted on examining the DNC server. The refusal of the DNC to turn over their computer hardware was prima facie evidence that there was no crime. It was the equivalent of someone reporting a burglary but never allowing the police into the house to investigate the alleged crime scene. The lack of cooperation on the part of the alleged victim is all the police need to determine that no crime was committed.
The most important angle to that story is how the FBI got involved in the first place. Did they get involved based on media reports, or did someone at the DNC actually report the matter to the FBI? If it was the latter, then this would clearly be just one more case where a DNC operative may have lied to the FBI.
I side with your conclusions. However, they don't have to turn over the hardware. If they want to be co-operative, they could have provided an image of the server or pertinent log files. This was a bulk release of emails, so it was either insider or a hacker that gained admin privileges. It could also be something like the Equifax hack when a crucial system didn't get patched. I haven't heard of any reports that puts that theory high on my list.
My recollection is there was a public pissing contest between the FBI and the chair of the DNC at the time, when this last garnered any press coverage. The FBI claimed they contacted the DNC when they first heard about the potential hacking, from where I don’t recall. The FBI claimed they were ignored, if not rebuffed. DWS claimed the DNC was never contacted, at least not at the appropriate levels.
Ultimately, the only suppposed evidence put forth that they were hacked, came from a 3rd party company called Crowd Strike, indicating the FBI was pretty much cut out from investigating directly, at any point in time. Around this same time, Wikileaks revealed US agencies such as the CIA had the ability to mimic attacks as originating from elsewhere.
The DNC discovered the leak/hack in late April 2016. The DNC/Hillary Campaign/Perkins Coie hired the cybersecurity firm Crowdstrike and it began monitoring the servers on May 5th. Debbie Wasserman Schultz did not get around to informing other DNC officials of the leak/hack until July 2016, and only because the story uncovering it was about to appear in the WaPo. In June 2017, 0bama's Director of Homeland Security, Jeh Johnson testified before congress that the DNC rebuffed FBI help, and in her book published last month Donna Brazile wrote that both Susan Rice and Eric Holder told her the DNC refused to cooperate with the FBI. My guess is that the FBI only learned of the matter in July 2016, with the publication of the WaPo story.