Unfortunately, the answer to all of these questions is up in the air right now. There have been some indications that the current RDA for vitamin D is too low, like an increase in rickets among dark-skinned populations in northern cities. The current RDA is at 200 IU for women between 19-50 years
, but if you meet that RDA and your levels are consistently so low, experimenting with those prescription meds to figure out what you need to take to have blood levels in the right range is really the best strategy.
There's some reason to associate low D levels with preeclampsia, but no one is certain yet if this is correlation or causation -- maybe there's the same cause underpinning both problems, especially since some autoimmune conditions often mean low serum D levels, and autoimmune conditions predispose to preeclampsia. (Plus, I have one of those conditions plus normal vitamin D levels. So it's weird.)