  Phase diagrams in the plane of $r_A$ (the average ionic radius, related to
one-electron bandwidth $W$) and $\sigma^2$ (the ionic radius variance,
measuring the quenched disorder), or ``bandwidth-disorder phase diagrams'',
have been established for perovskite manganites, with three-dimensional (3$D$)
Mn-O network. Here we establish the intrinsic bandwidth-disorder phase diagram
of half-doped layered manganites with the two-dimensional (2$D$) Mn-O network,
examining in detail the ``mother state'' of the colossal magnetoresistance
(CMR) phenomenon in crystals without ferromagnetic instability. The
consequences of the reduced dimensionality, from 3$D$ to 2$D$, on the
order-disorder phenomena in the charge-orbital sectors are also highlighted.
