Date of hypothetical medieval exemplar? [Thought experiment]

[Koen G]

I believe the VM images ultimately go back to a number of pre-medieval sources. This thread is NOT about that. It's about what could have happened in between. For any material to be carried from let's say the first century to the fifteenth, it is reasonable to assume a number of intermediate steps. Copies, additions, reinterpretations, updates, translations, and perhaps in our case, transcriptions.

We know that the physical object has been created in the early 15th century. This is one of the few things we can build upon with relative certainty. But, as you well know, any contents of a medieval manuscript, whether it be text, image or both, are not unlikely to have existed in some form earlier. Copies were made, traditions formed, fragments from various sources combined, altered, commented upon, and so forth.

There are a number of images in the VM which are medieval without a doubt. The crossbow, the castle walls on the rosettes foldout... I would like to find out when these were added. For example, is it possible that the VM was copied from a 10th century work? 

I will phrase this in a very concrete question:

What is the terminus post quem for a hypothetical medieval document from which the VM was copied?

• For the imagery?
  
• For the script?
  

Obviously things that pertain to the physical makeup of the manuscript are unlikely to help us much here, since they depend on the availability of materials to the 15th century scribes. But apart from that, which parts of the manuscript unambiguously point towards an early 15th century context?

I also have no idea when the script could have been given its current form. Are there indications that the script must have been a 15th century creation? For the record, this is not an idea I'd oppose, since to me this would mean that a source text was transcribed in the 15th century, which would be extremely interesting to know. But if this glyph set might as well have been a product of the 13th century, then that's interesting as well.

So has anyone studied any of this?