A bit more progress. I ordered a 12mm rounder bit from Amazon, which quick measurements tell me should come in handy to conceal the botched edges.
Today I stuck some 180 grit paper on a straightedge, and trued the joining edges of the maple cap. They match nicely with no visible gap when held up to the sunlight. I'm encouraged again, having felt a bit down in the mouth about my router antics.
I then built a jointing jig from scrap, and screwed the whole affair to my Workmate. Non stick paper was applied to the floor of the jig, the edges of the cap were coated with Titebond and wedged, and clamps and screws were applied generously.
This process proceeded without any issues arising. I'm going to leave it until tomorrow, and we will see how it looks. I ended the day by sanding the edges of the body with a mouse sander and 240 grit paper. Looks better already. There is plenty of sanding and tweaking to do on the inside edge of the horns and at the waist, to get it all neat and tidy.
Here are some pix: