9. Why is it impossible just to take out something from the Quran and leave the rest? Or even have a religion made up by ourselves?

Allah says in the Quran: { And if your Lord had pleased He would certainly have made people a single nation, but they shall continue to differ. }
Sura Hud (11) verse 118

A divine revelation which inaugurates a religion as such is - what in modern speaking might be called - a package. It's a coherent pattern, an organic whole, in which each part fits precisely with any other part and in which all the parts are interdependant.

This is why, you cannot take the best parts of each religion and combine them in some sort of super-faith. Moral laws differ, though less than many people suppose, between the religions, but these laws always make perfect sense in the context of the basic doctrines, the theology and cosmology of the faith to which they relate.

God wills variety, because His infinite richness cannot be fully reflected in any earthly pattern. - How could it be, given His infinity and our limitations?

We don't need to ask how these differences can be reconciled, we are finite beings and there is a boundary set for our understanding. So the Quran commands us to be patient, we will know the answers at last. Meanwhile we seek enlightenment in the Origin of all light, we seek help from the Source of all help and we put our trust where it belongs, not in the cod-man within us, but in God beyond us, not in me with a capital "M" , but in the book, given for our guidance and as a mercy for mankind.