Proposed Manuscript Chronology

[MarcoP]

could the gradient also be in reverse, first Bio and last Herbal A?

yes, absolutely the gradient could be in reverse, A path can be found but the order can't be specified from the data, I am making the assumption that Herbal A is the start and Bio is the end.

I agree with making that assumption. Though the binding order is unreliable, I think it still provides meaningful information:

• The only page with colored initials was bound as the first page (and it's Currier A).
  
• The only page with a mostly blank verso was bound as the last page (and the recto is Currier B).
  
• The binding order is partly consistent with Currier A/B: strongly A at the start, dubious pages in the middle (Cosmo/Zodiac), strongly B in the second half (Bio, Stars). The attached graph plots page numbers vs MDS1 as provided by npcompl33t (You are not allowed to view links. Register or Login to view.): correlation -0.66. I hope I haven’t messed up things too much.
  

In my opinion, this evidence points to “A before B” rather than the reverse.