Re: Keile/Kayla/etc - Yiddish?
To the meaning "black" – this is possible by relating it to the German word Kohle "coal", and the shift in the vowel looks plausible. There is also the adjective kohlschwarz "black as coal".
Next association: Keiler "wild boar". In High German the feminine counterpart would be Sau "sow" or Bache "sow having piglets", so no natural connection of this word to a feminine name.
To ETA2: I wouldn't consider Geila as a cognate because Yiddish usually preserves the High German state of consonants, and hardening of G to K occurs only in the Southernmost dialects of German, but Yiddish comes from the middle-west German dialects. In fact, when considering G->K shifts, Abigail would be another candidate origin.