Your depth of response and thought is great but your argument is flawed when you reckon that Holland can cover 1-7. Holland can play wing, centre and a bit of fullback if required. He cannot cover 6 and 7.
He covered roaming 5/8 against Cows when JMK went to hooker and as far as 7 is concerned,when you are a roaming 5/8 you infact become another extension of the 7.
A roaming 5/8 doesn't play the way we've been used to for the last 20 years,it's a lot more technical and subtle.
A 5/8 can interchange with the half in a strategic play that's usually called the 3 or 4 pass shift.In this strategy all spine positions become interchangeable.
The objective is to work the ball from sideline to sideline,to try and catch the defense out,as the spine creates a channel or an overlap.
Fast pass shift is a lot harder to defend against then the current slower predictable edge play,that relies on line breaks and tackle busts and the involvement of a 2nd rower's big man’s hands.
Currently Storm and Dragons are occasionally executing this strategy.
First they work say the right side using a 2nd rower,center or wing to try and drag the defense as close to sideline as possible.
Then the center,2nd rower or wing goes to dummy half instead of hooker.
Munster the 5/8 goes to 1st receiver in place of the half (so now 5/8 effectively becomes the 7) and shifts it left to Smith a hooker,who is at 2nd receiver instead of 5/8.
Smith shifts it to Croft the half,as the left side 2nd row runs decoy.
Croft shifts it to Slater the fullback,as the left side center runs decoy.
Slater shifts it quickly to Ado Carr.
If the ball shifts fast to the left and if the decoys time their runs correctly,you are guaranteed of a channel or an overlap.
As the ball is shifting left,you keep the defense guessing by occasionally taking on the line,or quickly shift to the opposite wing.
So as attacks evolve all spine positions change to specific strategies.There are loads of other strategies that you can study where the 5/8 foils in and replaces the half,or interchanges with the fullback,most are done so quick that most people fail to notice until it becomes the norm which usually means it's time for a change.