Presume James hasn't seen a 4wd sump hence not knowing about the extra drain under the kidney box
The only issue with using this drain from the separator is the bore size, I'm told it's not big enough and under heavy breathing the separator could overflow, IMO the engine would have to he well worn fir that too happen.
I it's fine to breath from both sides of the block, I'm am doing this during my engine build on my race car

Steve