Great example! Thanks for sharing.
Upon an error you could deploy the pretty-diagnostics tool by @awvwgk (a discussion is available in the thread Exploring first class error messages for Fortran). I wonder if there is a preprocessing trick, which would help share the namelist arguments between the parser and the error reporting.
I think it’s exactly what @urbanjost is trying to suggest as an enhancement, in order to avoid writing/copying the boiler-plate read/parse subroutines.