GWH
I'm afraid I must support the submission of my learned friend MR HD.
Comment on your previous point - if I have understood you correctly...
The proposal for full mixed-mode is that, unlike TWASS, the runways are completely independent once BOTH sides are established. That is to say that one side can be doing 3nm (2.5 not approved during parallel ops) and the other side could be doing 5, or 6 or 8 or anything you like.
Where there is an imbalance of demand to the extent that one side has airborne delay and the other doesn't the traffic will be 'managed' on to the other runway (in ways I wont go into here) so that the spare capacity is given to the delayed side, balancing the delays.
The very reason for joining at High-side glide-path plus 5nm is that it then doesn't matter what the loading is on each side; both sides will always be able to operate independently because at the time the High-side needs to descend on its glide path any traffic on the Low-side MUST be already established.
The draw back of this is that, theoretically, if you are the only aircraft for the Low-side but there is a continuous flow onto the High-side, you must still join at High-side glide path range plus 5nm - you cannot join short. (In reality of course, that is not going to happen because there will always enough demand to fill both approaches and if there weren't then the traffic would be balanced up so that there would be.) Of course the High-side can turn in short, providing it is willing to stay 1000' above any traffic on the Low-side until Loc established.
So, because your suggested blunder would always be occurring outside the High-side Glide path range (because that is where ATC will vector it to, even if number one) it will always be vertically separated from any traffic on the other side that not yet established and cannot affect the descent of the traffic that is established. Speeds difference, catch-up, overtake - no problem. This is why TWASS doesn't work; we must go for the full option because it is the easiest, safest and highest capacity but carries this one penalty. Clear as mud eh?
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