Voynich decoded completely

[-JKP-]

Doing this, one finds that by comparison P.hybridus'  leaves are of quite a different structure and class from those shown in that Vms picture.

No, they are quite similar. Even the tears in the edges of the leaves are one of the distinctive characteristics of many species of Petasites (this tends to happen on the larger leaves and later in the season when it goes to seed). And for the record, the rounded shape of the VMS  leaves is more similar to some species of Petasites than they are to banana.

P. hybridus does not have root remotely like that displayed within the Vms' picture..

It's quite a variable species. In some regions the root is more rounded like the VMS plant. In fact, in the regions in which it is round and swollen like the VMS root, it is sometimes bright red (even redder than in these examples):

Nor is P.hybridus a plant with layers of hollow stem - plainly shown in the Vms' picture. 

You are mistaken. One of the distinctive characteristics of Petasites is that the petioles (the stem part that attaches the leaves) are hollow. (Source: "A botanical and pharmacological description of petasites species", Tys, Szopa, Lalak, Chmielewska, Serefko, Poleszak, Curr. Issues Pharm. Med. Sci., Vol. 28, No. 3e, Pages 151–154; 2015).