Haleivi, my father z’l always said, if you take every midrash at face value, you’re a tipesh, and if you take none of them at face value, you’re also a tipesh. So if there’s a stira in midrashim, I wouldn’t lose sleep over it.
Nonetheless, I have to ask, if “pesach” was kept by Avrohom Ovinu in Tishrei, the more fundamental question to ask isn’t about when he celebrated, but what was he celebrating? “asher posach” related solely to the malach hamoves passing over the doorways of the Jews because they marked them with the blood of the korban pesach; “Vehigadeto levincho…ki bayom hazeh osoh h’ li betzeisi mmitzrayim,” but he was before, and couldn’t say as we do he was liberated from a place where he wasn’t enslaved, and his ancestors werent enslaved. Matzo – he could have eaten it, sure, it but it wouldn’t have been in memory of a swift departure wth no time to rise.
All of these mitzva rationales are of fundamental importance to us, ex post facto, but would be meaningless before the event, so what reason would he have had to celebrate davka “Pesach” at any time of year?