Do abundant supplication (du'a), seeking forgiveness (istighfar), and reading the Qur'an improve one's character, or are character and disposition inherited or a result of environment and upbringing, requiring effort to change them? And is attributing mistakes to Satan an evasion of responsibility?

The phrase you mentioned can have two meanings:

First: If you mean that abundant supplication, seeking forgiveness, and reciting the Quran have no effect on improving one's character, then this is false. The Prophet, peace be upon him, used to pray to Allah to improve his character and to seek refuge from its evil. Recitation of the Quran has a great impact on character, as seen in the state of the Prophet, peace be upon him, during Ramadan.

Second: If you mean that mere supplication and recitation are not enough without striving against the self, then this is true. The deficiency lies with the supplicant and the reciter, not with the supplication and recitation themselves. The effective elements are sincere supplication and humble, reflective recitation.

As for your statement: "We should not blame Satan for all our mistakes," this statement is incorrect if it assumes a necessary correlation between attributing evil to Satan and escaping responsibility. There is no such correlation. The Quran is full of attributing evil to Satan, because he is the origin of evil and whispering. More than one Companion said: "If it is correct, it is from Allah, and if it is wrong, it is from me and from Satan." The servant's attribution of evil to Satan does not mean shirking responsibility, but rather an admission of his heedlessness and an awareness of Satan's enmity, which necessitates constant vigilance and seeking refuge with Allah.