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Originally Posted by azzzy 
1. I am thinking of using Roxul Rockboard. It comes in various densities from 4 pcf to 8 pcf. From what I read I understand the denser the better. Is there such thing as too dense? What would be the optimum density for the absorber material for low freq absorber?
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See here:
http://bobgolds.com/AbsorptionCoefficients.htm
Looks like at a 4" thickness, 3.5-4 pcf mineral wool is the sweet spot.
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2. The edge effect. Since exposed edges of the absorber contribute to more absorption would it make sense to place two 2'x2'x6" squares (one above the other) with an air gap vs. one solid 2'x4'x6" absorber? What would be the optimal air gap? And if the above is true why not split it into 4 1'x2 pieces and place them one above the other with gaps between them? Anybody done that?
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I wouldn't bother as that is a side effect essentially. You want to focus on having usable surface area.
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3. Placement. I read that the common space between the wall and the absorber is 16". Does the effectiveness of the low freq absorber diminish once it's moved farther from the wall? If not, would it make sense to place a couple of low freqs absorbers somewhere in the middle of the room (in addition to the absorbers in the corners, of course)? Or the 16" gap is the fartherst the absorber should be away from the wall?
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I would not consider that common at all. 16" is used for measurement purposes at times. Usually a 2-4" gap is a very good place to be to have excellent performance AND still be able to fit yourself and maybe a chair in the room.
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4. If I have a bass trap in the corner does it have to touch the walls or is it better when it's farther out into the room? Can I place something relatively thin behind the bass trap (like a PA speaker stand - a 3" thick tripod)?
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You don't HAVE to touch the walls (though you should be in a very close proximity to the corner), but for space efficiency sake, you typically do. You could place something behind it, but generally you want to go floor to ceiling, so the space is not usable in any event.
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5. Different densities are optimal for absorbing different frequencies. Does it make sense to make an absorber layered with different densities? For example, using Roxul: ....
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No. First, what you describe will not do what you are thinking it will. And even if it did, you want broad band absorbers, not tuned traps.