Israel hosted the UN investigation, both presenting to them their evidence and letting them travel around in Israel to the impacted areas, and letting them go around on their own including visiting the West Bank and meeting with Palestinian representatives.
They did make some effort, apparently, to dictate what the parameters of the investigation and report needed to be, which the report authors rejected which made the Israelis mad. Then they did the investigation and wrote their report anyway.
I honestly don't know what you mean by "blocking the UN investigation," but I suspect that it has to do with the Israeli government's non-cooperation with the investigative team at times, and rejection of a more thorough investigation, which I suspect was caused by them wanting to be able to lie without anyone investigating their lies. To me, that's a positive thing about the report and investigation, not a negative thing. If it's attempting to be objective in a way which angers Israel, including debunking some Israeli lies, then good.
Yeah. This is their press release with the quick summary, and this is the full report.
Yeah, agreed. I'd imagine there's a lot of variability.
The report talks about what they found interviewing returned hostages on page 18:
The mission team reviewed incidents of alleged sexual violence related to hostages in Gaza. Based on the first-hand accounts of released hostages, the mission team received clear and convincing information that sexual violence, including rape, sexualized torture, and cruel, inhuman and degrading treatment occurred against some women and children during their time in captivity and has reasonable grounds to believe that this violence may be ongoing.
Based on first-hand accounts of released hostages there are reasonable grounds to believe that female hostages were also subjected to other forms of sexual violence.