Naming of ester alkoxide
As you have already mentioned, the key to the answer of this question is the seniority order for classes that are expressed by suffixes.

Esters are senior to hydroxy compounds (including alcohols). Therefore, the preferred IUPAC name for the corresponding glycolic acid ethyl ester with an intact hydroxy group is ethyl hydroxyacetate. (Note that according to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the locant ‘2’ in ‘ethyl 2-hydroxyacetate’ is not cited.)



However, anions are senior to esters. Therefore, the anion formed by subtracting a $\ce{H+}$ from the hydroxy group of ethyl hydroxyacetate is named as ethanolate. The ethanolate is substituted using the usual principles of substitutive nomenclature. The prefix ‘oxo’, denoting $\ce{=O}$, is used to indicate a carbonyl group when the group cannot be cited as suffix. The group $\ce{-O-CH2-CH3}$ is indicated by the prefix ‘ethoxy’. This leads to the systematic name 2-ethoxy-2-oxoethan-1-olate for the anion and potassium 2-ethoxy-2-oxoethan-1-olate for the salt.