I don't have any more specific information. I would think Ethlyn is just a shortened version of Ethelyn, which was on the SSA top 1000 list in the USA almost continuously between 1880 and 1938, and is just a blend of Ethel
I suppose there's a chance that whoever first created Ethelyn and Ethlyn deliberately meant them to be shortened forms of Ethelinda
, an Old English name which had a minor revival in the UK in Victorian times. One certainly can find intermediate forms like Ethelind and Ethlind in genealogical records now found on the Web.