How to configure the alpha to stay seamless and avoid the edges marked in red?
Is your alpha actually seamless? It looks as if it might not be.
My image is with the Alpha 62 from the Alpha palette.
Yes. I create alpha seamless using the wrapmode (set in 3) in Brush Palette/Curve, using the plane Tool converted in makepolimesh. If you look carefully, the texture fits perfectly if you disregard these divisions that have happened. These divisions that I marked in red happened when I decreased the scale of the alpha.
Can you post the alpha so I can test?
oops. disregard