Female ames ending in -lyn have been trendy on and off for most of the past 100 years, at least. Somethinglyn names come in all shapes and sizes.
Ethlyn is probably, as you suggested, related to
Ethalyn, ie
Ethel and
Lyn. Or it could just be a random combination of the then-popular sound 'Eth' with the trendy suffix 'lyn'.
Cleveland Kent Evans may have some more specific info on Ethlyn?