a) the convective cell is over a maritime area instead of a continent; b) the diurnal cycle is strongly affected by land-sea breezes; c) the main heat source is over the equatorial divergence belt

The density (mostly caused by temperature) of a baroclinic air mass is not a function of pressure alone. I.e., density can change even if pressure remains the same. In a barotropic airmass changes in density are caused by changes in pressure (often associated with changes in altitude).

Walker circulation is a zonally-oriented cell in the tropics that moves air upward in convection cells, east (or west) toward a jet stream minimum, where the air sinks to the surface and returns to the convection cell via surface winds driven by pressure gradients. Hadley circulation is a meridionally-oriented cell that begins in the tropical convection cell like the Walker circulation, but moves poleward with a low pressure cell pulling it to the left and the Coriolis force pulling it to the right (opposite in Southern Hemisphere), until it sinks beneath the subpolar jetstream and is returned to the low latitudes along the surface down a pressure gradient.

El Niño is associated with weak convection over the maritime continent because the pressure at Darwin (northern Australia) becomes higher relative to the pressure at Tahiti (the Southern Oscillation). Since the cold surges of the winter monsoon seem to enhance convection in the tropics, if there are fewer cold surges, perhaps convection is weakened, increasing pressure in the Indonesia area and perhaps contributing to the onset of an El Niño.