One more thing on these forced sales - it appears to me that when the margin department forces a sale to meet a margin call, the DO NOT have to go out through level 2 (to get the best bid) to do so.
I have had several stink bids filled at Schwab where my bid price was actually lower than the price published on level 2 at the time. From this I can only surmise that the margin department can (and does) go to open buy orders within their own customer base when doing a "sell at market" on these dumps without having to market the order through level 2.
Perhaps someone who has worked in the margin department at a brokerage could provide more color on this, but that has been my actual experience.