// In the youtube link provided by OG, the supposed lawyer is effectively making the same arguments as [Lord Pannick KC] ... //
Barrett even cites Pannick early on, in a passage below that I quote verbatim:
// None months ago, probably the most respected barrister in the land, who beat Boris in the prorogation case [and also the first Brexit case, on Article 50 notification], which caused Boris a huge amount of embarrassment and political difficulty and all the rest of it, who is in the House of Lords, who is a highly highly respected non-partisan figure, wrote to the Committee and advised them that they were messing up... //
The reference to Pannick being "non-partisan" is particularly noteworthy. It is, in fact, wrong. Pannick was on Johnson's legal team throughout these proceedings. He was therefore paid to provide arguments that favoured Johnson's position. He is indeed a highly-respected lawyer, and it's therefore not surprising that he tried to do his job of defending Johnson as effectively as possible. So that's point one: Pannick's opinion was not written, despite what this video appears to imply, from a "non-partisan" perspective.
It's important in that context to address the Committee's own reply to this:
https://committees.parliament.uk/publications/28816/documents/173975/default/ . As fa as I can see, the video doesn't do it -- some token acknowledgement that the Committee isn't a legal body, that it in fact sits at the top of the Constitution, that MPs in fact must be their own "judges" as a consequence of that, but then moves on.
From the link above: // We are grateful for the advice we have received, in compiling this report, from the Clerks of the House and Office of Speaker’s Counsel, and from our legal adviser Rt Hon
Sir Ernest Ryder ... //
Ryder himself is a respected barrister and Judge, and while the same caveats apply to him in the sense that he'd be engaged in putting the case for his "clients" (in this case the Committee), it's still important to recognise that legal opinion on the Committee's approach to this enquiry was not one-sidedly against it. More relevant is the advice "from the Office of Speaker's Counsel", since at some point in OG's video (about six minutes in), the interviewer asked if the Speaker should be responsible. Well, he was. And he (through his office and Counsel, and also directly) approved their approach fully. As the response says:
// The view of our impartial legal advisers and Clerks, which we accept, is that his opinion is founded on a systemic misunderstanding of the parliamentary process and misplaced analogies with the criminal law. //
But, finally, the condition on whether the Committee's report and approach are acceptable is entirely on whether or not Parliament as a whole votes to approve it. This they will consider (and almost certainly do) on Monday. Assuming that Parliament does approve it, the Committee's approach was within the rules of Parliament and therefore Constitutional.