Hmm, would use a 32' flue in the middle section anyway - not (just) for the low B. Why did he write it? didn't the french have ravalement only for the reeds?
I would use a 'grand plein jeu' for the middle section, because to me it resembles more to plein-jeu pieces than to the GrandJeux parts in for instance the Marchand GrandDialogue.
The final candenza always strikes as me a bit disappointing; the notes going up on the low D and then the trill almost an octave lower.
Like someone who speaks more and more agitated and then suddenly gets himself together (like "oh well, why bother ...).
Strange, someone?