Ok, I have been doing some more research and I have talked with a friend who had some interesting thoughts after looking at the pictures I sent her. She clarified to me something I had noticed but didn't put together. She felt that the round sukashi seems not to have been cut at the same time as the "gourd" which I believe is original to the tsuba, but put in when the nakago-ana was enlarged.
The sides of the gourd are perpendicular to the face of the tsuba but the sides of the round sukashi slant in and are not very skillfully done. The nakago-ana also slants in the same manner although on the opposite side (not sure which is the omote and which is the ura on this one). The nakago-ana has also been enlarged and not in a very skillful way. The inner edges of the round sukashi also are not the same as the inner edges of the gourd, but rougher and not as well cut and similar to those of the nakago-ana.
If this is so, then I am going back to my original thought that this is a Ko-Tosho from early to mid Muromachi that originally only had the "gourd" as decoration. The size (7.8 cm diameter and 3 mm thickness at the seppa-dai and 1.8 mm at the mimi) and the single, small ko-sukashi all fit. Some time down the line, the owner, decided to have it put on a different sword and had the nakago-ana enlarged and the other sukashi put in at the same time. The nunome would also have been added at his time to embellish the appearance (as evidenced by the decoration going around both sukashi and not interrupting it, which it would have if it had been applied prior to the round sukashi). Added nunome is not unheard of (see the second tsuba down here: http://www.shibuiswords.com/EDLcollection.htm which also has a bit of nunome left on it).
Obviously, this is all conjecture, but it seems to fit better than what I have been struggling to reconcile. I would still be interested in any thoughts or comments. I will say that this has been a very pleasurable exercise so far. :D