1. Not sure, with the 101 delegate system, I think the theory is the average user will not have to download the entire blockchain, I see parts of it are discussed here https://bitsharestalk.org/index.php?topic=9204.0, maybe someone else can give a better answer.

2. There is a requirement for all shorts to cover every 30 days, so even if you bought an asset in a low volume market you should be able to get fair value for it. I imagine a market can also be discontinued with notice if there was really low volume.