The trouble with cleaning with a dry paintbrush is dust does not stick to the brush. You end up just moving the dust around. Use a microfiber brush or one that dust can cling to. or can compress air to blow the dust out.
As to the questions, a brush can sometimes loosen a dust film from circuit boards but won't pick it it. Use in conjunction w/ compressed air to get things all spiffy clean and make sure that brush is for electronics otherwise can create static that might fry sumthin .... brushes that 'attract dust: use either static or goop to make it stick and neither is a good idea around electronics.
A quick "dry" blow is better than nothing, no spit, please!
Any type of fluid is bad for the motherboard. If you inadvertently do spill moisture on the motherboard, don't use a hair dryer, just leave the motherboard to dry on its own for a few days. you could use a hair dryer but not on heat settings just cold air only.
You could try an old toothbrush (no paste) but don't scrub, just gently brush the areas and be careful not to knock any of the capacitors, they are held by simple wires in place, easy to break them from their moorings.
Don't forget to vacuum your room. There should be no dust near your PC. Also, a good idea is to raise the PC case off the floor or carpet area by about 4cm or more. Use a board or some tiles. Dust is attracted to metal and anything that exhausts/inhales air.
A slightly damp cloth picks up dust nicely partially the blue microfiber ones, baby wipes also work quite well but tear easily if they catch on anything. blowing the dust can cause it to get blown into places you can't get it out of so its something to watch for. :-)