I can't really make sense of the
HS2 2b plans with regards the Leeds line provisional allowances. Do I have it right that the "Manchester to Leeds junction" marker points to the Eastbound line only which would run along the top of the "box" in one direction and land in the middle of the HS2 running lines just before the station S&C in the other? Then the Westbound would do the same, but inside the "box"?
Delete "in the middle of" from your description.
HS2 trains terminating at Manchester would approach and leave through the box and use middle platforms at Piccadilly High Speed.
Trains from Birmingham or Liverpool to Leeds, reversing at Manchester, would approach on HS2 through the box, use southern platforms at Piccadilly High Speed, leave on the southernmost track, and go over the box on the way out, crossing over to the north side of the line towards Leeds.
Trains from Leeds to Liverpool or Birmingham, reversing at Manchester, would approach on the southern track from Leeds, go through the box alongside HS2, into platforms on the north side of Piccadilly High Speed, and then go through the box on their way out onto HS2.
I've seen diagrams and will try to find one for you, but I often have trouble navigating the jungle of HS2 documents.
Edit: See figure 4 in this document:
It's on page 28 as shown by the document's internal page numbering, or page 31 as shown by the PDF reader in my browser. It's looking from the east towards Piccadilly. The words "eastbound" and "westbound" are swapped in the key to the diagram! Green is really towards Leeds and red is from Leeds.