Getting good coverage with white is always problematic.
Tamiya flat white is actually not too bad, but it will require multiple (4-6) coats.
If your finish is grainy, I would think that you're not thinning the paint sufficiently. I can understand that you may not wish to thin the paint too much so as to obtain maximum coverage, but Tamiya paints go down smoother when thin. I'd be looking at about 1:1 paint/:thinner.
Your VL is something of an air hog and typically requires higher pressure than other AB's. Though I don't use a VL, I think 15PSI is probably below it's optimum working pressure. Reducing the pressure even further, with insufficiently thinned paint may actually promote tip dry and poor paint flow.
Though it sounds counter-intuitive, I would thin the paint some more and increase the air pressure (say 18-20PSI), but moderate the paint flow by using less trigger.
Apply thin coats, allowing each to dry before overcoating, until sufficient coverage is attained/
As far as retarders are concerned, you don't have to use Tamiya's own. Modellers have been using acrylic retarders, for example, those by Golden, Liquitex, Jo Sonja's, for many years, long before Tamiya offered theirs.