Taking those one by one:
Hooker:
Throwing is definitely key I'll give you that, but also a line out can go to pot for any number of reasons
No. 8:
In reality how often is ability to control the ball at the back a genuine game changer? I can't think of any match where that has been a key factor (unlike the lineout, or front row battle)
9 / 10:
I agree on these two, for me these are the actual key positions
Fullback:
Equally if one of the wingers can't catch you can just kick to him, it's not unique to FB, there are three players patrolling that backfield
(For what it's worth I think you underplay the significance of a dominant scrum in your counter example too.)
My mind remains unchanged I'm afraid! I think if you "reverse engineer" the question, ie start from the assumption that the "spine" is significant and work backwards, you can come up with justifications for the concept. However if you start with the question "which positions (if any) are categorically more important, across all leagues and teams, than the others?" then I don't believe you can build a compelling case for any except the half backs.