Is it permissible to leave investment certificates in the bank and dispose of their returns for the benefit of Muslims, after becoming convinced of their unlawfulness, knowing that they were purchased based on a previous fatwa allowing them?

These certificates are not permissible to keep. They must be returned to the bank and their value reclaimed, for Allah the Almighty says: ﴿O you who have believed, fear Allah and give up what remains [due to you] of interest, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged.﴾ It is not permissible to wait until their redemption time to receive their returns and spend them on Muslim welfare projects.