Is prayer valid with the first ablution if a person intended to renew their ablution, then changed their mind and sufficed with the first? And does the first ghusl remain valid and can prayer be performed with it if a person intended to perform ghusl for janabah a second time then retracted from completing it?

Renewing ablution is recommended for every obligatory prayer. If one begins it and then interrupts it, there is no sin upon them, as it is not among the supererogatory acts that become binding upon commencement. Interrupting it does not invalidate the first ablution.

As for ghusl (ritual bath) for janabah (major impurity), it has specific causes, and renewing it is disliked because there is no evidence to indicate its permissibility. Consequently, interrupting it is permissible, and it does not invalidate the first ghusl, and prayer performed with it is valid unless a new cause necessitating ghusl arises.