"A number of groups... asserted it does NOT look like a HIMAR strike".
Really? What I have not seen, nor have you cited, is even one specific expert.
This is like presenting to a jury at trial... "Ladies and Gentlemen... we have expert witness which exonerates the accused! But we are not going to tell you who the expert witness is, and will not provide their credentials, will not present them for cross examination, nor will we provide their specific testimony. But trust us, they exist and totally back us!"
I am seeing a pattern here where "evidence" which fits views you hold is accepted on faith, while that which does not is dismissed as "Russia's narrative".
As to admitting "independent" investigators to the site. It is doubtful whether a third-party observer would be independent - and that anything they discover would not be discounted if non-conforming. This topic was already covered. It is a war zone and crime scene. These zones routinely controlled. The Ukraine side also routinely excludes access to attacked sites - and you have not mentioned it at all. Does this mean they are hiding something?
Please note that the range of explanations I allowed for DOES include a Russian False Flag operation.
But what you call "evidence" (like supposed expert blast analysis), I do not, hence I see no basis for definitive conclusion one way or another.