"all" is not part of the pronoun. It is an additional term which adds emphasis or implies membership in a collective. "You all" is valid English (hence why it appears in the bible), but it has some additional implication which this sentence doesn't have in this German sentence.
I agree that "you all" should be accepted here. Or "y'all". I'm pretty sure I recall seeing "you all" accepted in other sentences as a translation for the plural second person pronouns.
Also, "any of you" would also express the plural aspect of euch, but unfortunately would imply the group could be separated.
That said, "you" is both plural and singular, and for the purposes of pleasing die Eule, that's all I use. English's lack of a distinct plural second person pronoun is unfortunate. That's why I'm glad we have "y'all" down here in the South.
Konnte is could. It's the past of Kann. Ich kann nicht euch finden = I can not find you. Ich konnte nicht euch finden = I could not find you.
While in english it's pretty much a like, in German it is not. All modal verbs are the same on that matter: Ich soll heute lernen = I should learn today Ich sollte heute lernen = I should have learn today.
könnte is Konjunctive II ('past' subjunctive), konnte is Präteritum (simple past).
Try here: http://www.germanveryeasy.com/modal-verbs
and here: http://german.about.com/od/grammar/a/konjunktivII.htm
for explanations of what that means.