Is what I said in prostration, supplicating, "May my limbs be paralyzed," while specifying not to return to one path of sin, but intending to return to another path of it, considered idle talk in supplication? And what is the ruling on what I did after returning to the sin I had covenanted with Allah to abandon?

Sincere repentance requires completely abandoning the sin, regretting it, and resolving not to return to it, not seeking alternative ways to commit disobedience. Whoever makes a covenant with Allah and breaks it is obligated to offer an expiation for an oath (feeding ten poor people, or clothing them, or freeing a slave, or fasting three days if unable to do any of the above). The relationship with Allah must be rectified through sincere repentance, for Allah is Merciful and accepts the repentance of His servants, and whoever strives for His sake, He will guide them to His ways.