They can be, if disclosure could compromise on ongoing investigation and of course it’s not always in their personal interest to disclose. There have been reports by third parties of grand juries being impaneled in the matter.
Again, not entirely true, they can be and are legally directed not to disclose if doing so could result in compromising an ongoing investigation. And as I mentioned, even if you can it doesn’t mean you would for various reasons.
I am unsure why you placed collusion in quotes. The Mueller Report called out collusion.
In any sense, the Mueller Report ended the investigation into the 2016 Russian meddling in the US election. President Trump, and Republicans, have now tried to pivot and claim that Ukraine was interfering the 2016 election. I am unsure of the basis of this, unless one is to believe a debunked theory that a DNC server is located in Ukraine.