As you said, all things are possible for God. The idea that God could not resurrect cremation remains is almost a denial of the power of God and gives humanity more power than God. After all, the idea that God cannot resurrect someone who has been cremated would mean anti-Christian individuals could deny the resurrection to Christians (and stop the will of God) by simply burning Christian bodies. It would mean that individuals who died in horrible fires would never be resurrected. It would also mean anyone whose body was decomposed to the point of total dust would not see the new Kingdom on earth.
As Catholics we do not place limits on the power of God or His ability to fulfill His will.
