1)
My guess is that whether you’re breathing air or seawater the equilibrium concentration of dissolved nitrogen will be the same. Though it probably takes longer to reach equlibrium breathing water rather than compressed air. Could be wrong though, maybe water at depth is less than saturated in dissolved nitrogen.

2)
Could also have a separate adaptation to moderate the formation of gas bubbles in the body fluids. Could even be the bubbles they exhale are excreted nitrogen gas.

I want my no-prize now dammit.
:)