I tried fitting it again today. I used some black permanent marker around the edges of the reduction and the inner tip itself, and then test fit it to see if I could see where it was binding. I could see a couple of minor scuffs when I removed it so, after a quick rap with a hammer on those scuffs, I tried it again. As before, I used a screwdriver to ensure the holes were aligned and voila! - it worked.
Strangely though, I couldn't get the original bolt to start threading, but the bolt for the straight-through baffle (the one supplied with the muffler) worked fine. I've taken it for a drive and all is well.
All this to say - "crisis" averted. I'll use it for a week or so and then switch it for the straight-through baffle again. Thanks very much for your concern and help!