endplay
I can only offer a personal opinion. Although I have been intimately familiar with the case, I cannot claim to speak on behalf of the campaign group, who have settled on this strategy. Their view may be very different.
There is a sense in which your question regarding IFR/VFR is irrelevant to this case.
The simple facts regarding the weather are that: Everybody standing on the land mass of the Mull of Kintyre was in cloud. This very localised hill fog surrounding the Mull was visible from the coast of Northern Ireland. Clear conditions prevailed to a position, undetermined but, extremely close to the Mull Lighthouse.
Whilst it is possible, though not provable that ZD576 impacted the hillside in cloud. This simple assumption does NOTHING to help us understand what the pilots could see, nor what was happening, in the moments leading up to that impact.
The tiny snippets of disjointed information available, simply aren't robust enough!
THAT is why your question regarding IFR/VFR simply won't help us to progress.
As far as I understand it the focus on the RAF's requirement of 'Absolutely no doubt whatsoever' is simply the 'key' to help unlock this unwarranted opprobrium.
Firstly because it is simple to demonstrate that the Wg Cdr who conducted the investigation, ALSO had a duty to find negligence where the weight of evidence demanded it. He knew the evidence perhaps better than anyone, and he made no such finding.
And secondly, because the tidal wave of evidence coming to light about the appalling state of this aircraft. at that time, means ANY reasonable person, could not find the pilots guilty of ANYTHING. Not even on a balance of probability. As demonstrated by EVERY SINGLE SUBSEQUENT INDEPENDENT INQUIRY! (and there have been many!)
Let right be done.