Six onion-roof towers supporting heavens

[ReneZ]

I also haven't seen any swallow-tailed merlons in his work, so I think it's likely the VM got its buildings from elsewhere.

This actually does not matter. When people copied buildings, they would change such details at will.
A perfect example of this is the walled garden in the Tractatus de Herbis tradition, which has straight crenellations in the southern manuscripts, which are changed to swallow-tailed in the Milanese Sloane copy.