Originally Posted by wogoga
OK, so you took what's effectively now a single source, split it into two paths of different length, and then combined the two paths after to destructively interfere.
Actually, this situation DOES result in 100% destructive interference. Sa1 cancels Sb1, Sa2 cancels Sb2. It's right there in your numbers. Treating the components separately doesn't change the end result.
The mistake that you are making is in thinking that this "adjustment" consists of anything other than BLOCKING the component of the waves which are not already in phase at the slit. The component of the waves which is out of phase doesn't get changed
to be in phases, it just interacts with the screen with the slit rather than what's beyond the screen.
But there is no slit in our astronomical case. The components which are not in phase don't get blocked, they aren't absorbed anywhere, they aren't reflected away. There is nothing which makes that light become coherent the way you are envisioning it.