I'm thinking that they should shoot the panel with single stage enamel and blend it only at the C-pillar.
I'm not expecting a miracle here as I know matching old paint is really hard to do. I just don't think using a clear coat is the way to go when just re-shooting part of a car that still has it's original paint.
I totally agree. Don't let them touch the original paint in any way. The quarter and C-pillar blend are already non-original so nothing to lose there. And the choice of single stage enamel is also a good one, just like what the factory used. Now, to find the paint and the person with the expertise to do the job.... Good luck