Gosh, it's a matter of taste, isn't it? However if you want to simulate 180 degree shutter and you had shot with a shutter that captured no blur, I would use 0.5. Since you already have some blur, shooting at 30 (degrees I assume), which is 1/6 of what you want (30/180), I would try (5/6)*0.5, or .416. Your mileage may vary.
As to grain: adding motion blur after grain will smear the grain, as you have correctly anticipated. Again, that look may or may not be what you want