Can someone give me an example of a Banach space $X$ and contractive projection $P\in\mathcal{B}(X)$ such that $\ker P$ is not a range of any contractive projection $Q\in\mathcal{B}(X)$?
The projection from the space $c$ of convergent sequences to the $1$dimensional subspace of constant sequences with kernel $c_0$. Any projection $c \to c_0$ has norm at least $2$ (cf. exercises to chapter 2.5 in AlbiacKalton). 

