I'm not exactly an expert on the nuts and bolts of Bitshares' internal working, so if you still have any questions I highly recommend posting to their forum: bitsharestalk.org
Nonetheless, here goes:
- Yes, the margin call price is dynamic and goes up and down with settlement. You can actually remove excess collateral at any time (which is nice!), and you can add collateral when you see the price moving against you.
- I'm not sure about called positions being taken off the order book once input. I doubt it.
- The original buyer of the asset from the person who borrowed it and put it up for sale is not involved in the margin call process; assets purchased can be held indefinitely.