1) what was Achan’s guilt and what did Avraham do for it? 2) how was the circumcision of Avraham and how did he come to this conclusion?

ב"ה Shalom, Your first question is answered by Rashi, and had you not learned Rashi or the Midrash you would not have been able to ask this question, since Achan is not mentioned in the Torah, but only in the book of Yehoshua. Rashi (Brereshit 12:8 ) based on Midrash Rabba explains, that Avraham had foreseen the future, that in the time of Yehoshua, Achan would sin. Therefore, Avraham built the altar between Bet El and the Ai in order to pray on behalf of the Jewish people. I don't fully understand what you mean by your second question. Hashem commanded Avraham to perform the Brit Milah and Avraham carried it out. Rashi (Bereshit 17:24) adds that Avraham due to his age was a bit afraid to do the actual circumcision ,therefore Hashem assisted him. All the best