I did experiment with feathers a few years ago. You can take a feather from the street using a plastic bag (no touching with bare hands until sanitized, the feather was from a crow, I think), boil it for a few minutes and then cut the end diagonally and make a slit starting at the tip about 10 mm long and that's it. Bought some ink at the stationary shop. It's surprisingly easy to write with a feather, at least when you do it slowly, no wonder quills were so popular. The problem is I now understand the basic mechanics, but I have no idea about professional tricks. E.g., whenever my self-made quill would split even slightly, I'd get an instant ink blot. Also, I'm not sure the viscosity of modern ink is the same as the iron gall ink from the XV century. So, at least for me it's not clear if it's possible to fully understand what exactly led to this or that feature of a pen stroke in VMS.
However, I fully agree that the same kinds of stroke defects, whatever caused them, consistently appearing in the text and the drawings strongly suggest the same hand and/or the same instrument.
However, I fully agree that the same kinds of stroke defects, whatever caused them, consistently appearing in the text and the drawings strongly suggest the same hand and/or the same instrument.