Two things:
1. Many years ago one of my work colleagues, a close friend who I spoke to every day, socialised with and collaborated on work with, passed away. That affected me emotionally and I am sure affected my performance for a time. On the other hand, at various (fortunately isolated) times in my career, work colleagues who I have no dealings with, and have never spoken to, have passed away. This has not affected me at all. It has never been demonstrated that the relationship between the players and Maddy Cusack was anything but in the second category, although I am prepared to be proved wrong on that.
Grief affects performance. We do not know whether there was any performance-affecting grief in the first place.
2. I did not see the 6-0 defeat against Arsenal or the 5-0 v Villa but I saw all of our other hammerings either live or on TV. I would still put Newcastle head and shoulders over the others, not just because how badly we played, but because that was the only game in which a number of players basically gave up and stopped giving forth the effort required. The game also had a cataclysmic effect on our season second only to the defeat at Tottenham. That was not the effect of grief. That was quitting on their teammates and the club.
And that is my last word on this topic.