The proseal idea is good. If you don't have any some JB weld would work - put some grease on the cleco. I would first try to see if I could put a stick in there to press the flange against the skin for riveting but be careful not to create a bulge.
I always drill out rivets with a #41 drill to avoid elongation. It makes things easier and you can still snap off the head and punch out the shank.
If you have 1 rivet out of all of them on a rib that is a bit less than perfect there won't be a safety issue. It really comes down to how OCD you are.
RV 4, with an engine...and other stuff
VAF dues 2016