Re: four questions about formatting of numbers ...

See "man -s3 printf".  The GNM_FORMAT_ part simply adds the "L"
modifier when long doubles are in play in addition to the g/G/e/G/f/F.

"E" vs "e": most places it doesn't matter, but for user visible places
it is a matter of tradition and Excel compatibility.

Why 2+ digits for exponents?  Don't know -- probably history and
tradition.  If you don't like the extra "0", it's an easy and clean
post-process step to remove it.