Yhyjyk

I have a bit of a confusion in regards to the Mueller investigation.

While it was undergoing, we were repeatedly told that "proving collusion" is pretty much impossible since there's no such thing as "collusion", legally speaking. The best one could hope for from Mueller's investigation is that he would highlight enough ... wrongdoings by Trump and his team, that Congress would end up impeaching him.

Nobody ever actually expected Mueller to do his investigation and end up concluding it with holy moly, I actually just caught Trump in the act of admitting he is a Russian agent! I even have it on tape! Woho! Collusion proved! ... rather, the expectation was always (at least from a Democratic perspective) that the investigation could bring to light certain facts that would make impeachment a viable topic of discussion.

However, now that the investigation has concluded, why is there then any possible debate on whether the report should be released to all members of Congress, completely unredacted?

Isn't that what the investigation was for? Isn't that why it's called an investigation? If the goal was to simply deliver a yes/no answer to congress with respect to whether Trump can be criminally charged or not, well, we all already new that was never going to happen. It was the contents of the report, and what consequences those contents may have, that was the primary goal with the investigation, was it not?

... why is there then any possible debate on whether the report should be released to all members of Congress, completely unredacted?

The U.S. House voted unanimously to release the Mueller report to the public.

• 
  House Votes, 420-to-0, to Demand Public Release of Mueller Report ~ NY Times, March 4, 2019
  

The President has repeatedly concurred, saying publicly that he has no objection its release.

So there's really no debate on the matter. Everybody agrees that it should be released.

The problem is that the report contains information that cannot be released by law and for national security reasons. The report also contains the personal information of tangential third-parties, and an effort is being made to protect their privacy. There's also an effort to protect the integrity of other investigations that are ongoing. So it will take some time to release the full report, which will undoubtedly be redacted. The Attorney General estimates by mid-April.

"As we have discussed, I share your desire to ensure that Congress and
the public had the opportunity to read the Special Counsel’s report.
The Special Counsel is assisting us in this process,” [Attorney General William] Barr wrote to
Senate Judiciary Committee Chairman Lindsey Graham, R-S.C., and House
Judiciary Committee Chairman Jerrold Nadler, D-N.Y.

Barr said the Justice Department and the special counsel are “well
along in the process of identifying and redacting” sensitive material,
including material that “by law cannot be made public,” “material the
intelligence community identifies as potentially compromising
sensitive sources and methods; material that could affect other
ongoing matters, including those that the Special Counsel has referred
to other Department offices; and information that would unduly
infringe on the personal privacy and reputational interests of
peripheral third parties.”

Barr said that he anticipates they “will be in a position to release
the report by mid-April, if not sooner.” A Justice Department official
this week told Fox News that the Mueller report is more than 300 pages
long.

Barr added that: “Although the President would have the right to
assert privilege over certain parts of the report, he has stated
publicly that he intends to defer to me and, accordingly, there are no
plans to submit the report to the White House for privilege review."

https://www.foxnews.com/politics/barr-to-release-mueller-report-to-congress-by-mid-april-if-not-sooner-will-not-transmit-to-white-house-for-privilege-review

The other issue is that, as a Special Counsel, rather than an Independent Counsel (Ken Starr), Mueller was still reporting to the AG. Ken Starr was independent from the DOJ and therefore could deliver his report directly to Congress.