PC Gold Icy Shadow Bow of Markmanship

Cold damage : 15-28 (req.11)
Damage +13% (while health is above 50%)
Markmanship +1 (18%)

PS: I also had two questions:
1) Can i add a bow grip of fortitude with out replacing the markmanship +1?
2) If the bow was req. 8 and i had 11 mark, would it do more damage or is it the same?

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