Hi Mark.
Second attempt at a reply, not sure what I did to the first one. You are certainly diving into this hobby/study, good for you. I suspect that the reason you have had few replies is that you have posed too many questions and without pictures there is a problem finding where to start. Hope some of this helps. There are some sixty odd smiths listed in Hawley who signed Munetsugu so pinning yours down may take a bit of effort. Koyama Munetsugu is as you say a big name, one of the biggest in the shinshinto era. Search the forum and I am sure you will come across him, especially the story about the naginata by him discovered in a Cornish barn not far from here...... (Can you hear the slight tone of envy?) This Munetsugu is usually associated with the Bizen tradition, not the Hizen which is very different, though like many smiths he was able to work in a variety of styles. There are several Hizen Munetsugu smiths, some working into the shinshinto.
My library is far from complete but there is a smith listed in Hawley, MUN34 who signed Mikasa sanroku ju Sanjo Kokaji Munechika. Food for thought, not a million miles away from the auction house translation. Whatever the outcome when you get the sword some photos would be great and might well help other members give you the information you are looking for. Of course the date you give might only refer to the sayagaki....? In which case once again pictures will tell much.
Looking forward to seeing the sword.
All the best