Will the Mufti who erred in his fatwa and followed it be held accountable, and will my prayer be invalidated if I acted upon his erroneous fatwa concerning the nullifiers of prayer?

The duty of a common person is to ask the people of knowledge about what is unclear to them. If someone they trust gives them a fatwa, then their responsibility is discharged. If a mufti (issuer of fatwa) strives to make a judgment (ijtihad), there is no sin upon him even if he errs; rather, he is rewarded. However, if he issues a fatwa without knowledge, then he is sinful. As for the prayer, there is no need to repeat it if the fatwa was on a matter of ijtihad and based on a reputable opinion. However, if the fatwa was based on an disreputable opinion, or one that contradicts a sacred text or scholarly consensus, then it is safer to repeat it.