Okay, I consider this a success even though I messed up pretty much all of it. I'll explain.
What I did wrong:
1) I forgot my sewing kit. That's why the collar is pinned on using the pins I fortuitously left in the bodice, instead of basted on.
2) I forgot to trim out the neckline. The original pattern isn't meant to be buttoned up all the way. I do want to be able to button this up so I meant to trim out the neckline 1/2 inch all around--I marked it but didn't cut it. I might start by trimming it out 1/4 inch all around to make sure I don't make the neckline too big, since I haven't done it already and still have the option of cutting out less instead of more. Anyway, that's why the collar isn't pinned up flush with the neckline. Obviously the "real" collar will match the neckline.
3) I need to trim the ends down. I forgot to account for the overlap in the front of the dress. But that's easy to fix.
4) The collar needs to be a bit smaller. This is pretty clownish.
However, the collar does fit the contour of the dress I think it would look OK if all that other stuff had been right, so I consider it a success. I'll fix that other stuff and make another test one, of course, but I think I'll have a collar made and attached in fairly short order.