Is reciting Surat Al-Fatihah seven times along with Surat Al-Ikhlas and Al-Mu'awwidhatayn after Friday prayer considered an expiation for sins, and what is the evidence for that from the Sunnah?

There is no evidence to prove that reciting Surat Al-Fatihah, Qul Huwa Allahu Ahad, and Al-Mu'awwidhatayn seven times after the Friday prayer is a cause for the expiation of sins. Rather, what has been reported is that whoever does so will be protected from one Friday to the next and it will be a safeguard for him from Satan.

Acts of worship are prescriptive and cannot be established except by a sound text from the Qur'an or Sunnah (definition), due to the Prophet (peace and blessings of Allah be upon him) saying: "Whoever introduces into this affair of ours that which is not part of it, it will be rejected."