> Ássa or Ása: which one would be the proper pluralization for the
> term gods,
As Eyja said, the nominative plural of <Áss> is <Æsir>. <Áss> is a
u-stem, so it forms its plural in <-ir>, which triggers i-mutation
of the root vowel if that's possible, and the i-mutation of <á> is
<æ>.
> and how does the (ing) or (er) appear at the end of a word.