I was also struggling with this part. By fortunate coincidence, I saw this thread on common contractions earlier. It mentions some patterns that I think are going on in the phrase:
- い sound can vanish after -て form
- れば becomes りゃ
So does it work if くれりゃ is a contraction of the ば-form of くれる? well which くれる? One listed on jisho that might work is 呉れる with the meaning, “to do to someone’s disadvantage.” The entry also notes, “Usually written using kana alone, after the -te form of a verb. Auxillary verb, ichidan verb”
I’m working backwards to make it work, I know, but here’s my reasoning: Coach is wavering on who to pick for starting setter. He sorta bemoans the fact that Kageyama is too self-disciplined / strict with himself (ストイック) to be the kind of person who is cocky about his talent or content to rest on his previous accomplishments. If Kageyama was that kind of cocky person, Coach could take advantage (付け入る) of that weak spot (隙) and not choose him as starting setter.
So I think the grammar is functioning like “to be in a state of being cocky-and it’s disadvantageous-hypothetical.” Or maybe easier to parse, “if he was disadvantangeously cocky” or “too cocky for his own good” or “cocky and it was working against him.”
I think something like that is going on.
I’m still confused about the extra あ though. I’m chalking it up to regional dialect / Coach extended the sound.