Polyelectrolyte (PE) grafting renders incredible "smartness" to nanochannels, making it capable of applications such as ion manipulation and selection, fabrication of nanoionic diodes, flow control, and so on. In this paper, we provide scaling laws that describe the dominant factors dictating the functioning of such PE-grafted nanochannels. Through these scaling calculations, we identity the phase space for the grafting density (a) and the polymer size (N) that simultaneously ensures that the grafted PE molecules attain a brush-like configuration and the brush height is smaller than the nanochannel half height. More importantly, we quantify in this phase space the conditions that allow decoupling of the PE electrostatic effects from the PE entropic (elastic) and the exduded volume effects. This decoupled regime is characterized by the fact that the brush height is independent of the PE electrostatic effects. In this decoupled regime, we next calculate the nanochannel ionic current for cases where the PE charge densities depend on pH (or pOH). Our results demonstrate highly interesting current inversion phenomenon originating from the triggering of "co-ion-dominated" ionic current.