...slight amount of slop actually allowed the rivet head to extrude back into the that tiny clearance, creating a 'nipple...
Actually that (above) was what I addressing -- with some fear and trembling -- here:
The reason it is important to put a flat surface against the rivet head is that the head will shape into whatever is holding it down when you pop the rivet. There is a huge amount of pressure shaping the rivet head as you pop it. As I said in the other post, what holds the rivet head down has to be a flat surface like the rivet gun tip or you have a mess.
In other words, if you use something that has an indentation in it, it will leave a raised surface that the car will whack it as it rolls by.
But, good, you found a work-around anyway and exterminated the "nipple".