Hill system with isotopologue
I can't point to a definitive authority, but in most of the papers I read, #5 other is used most often.  Specifically for ethylamine:

$\ce{^{13}C\,^{12}CH_7N}%edit$

I've always assumed the reason for this choice is to accentuate the presence of the isotopically unusual atoms.  I suppose a good test for my theory would be formulas with multiple, isotopically distinct atoms of an element such as iron, for which the most abundant isotope is not the lightest.

Hopefully other answers will be along shortly to point to something more authoritative, such as a IUPAC report, guideline, or document.