Is a worshiper sinful who mispronounces the word "Sirat" by omitting the hamzat al-wasl (connecting hamza), believing that this does not change the meaning, and relying on fatwas from some scholars that this error does not invalidate the prayer?

It is not permissible to drop the hamza of "an'amta," and the prayer becomes invalid by dropping it according to many scholars. However, there might not be a dropping of the hamza if you pronounce the alif. There is no sin upon one who is given a fatwa without knowledge.