OK, i'm not on the DS team, but I'll try to help out
2) DS does indeed handle the persistance of your GLOs, there is nothing you need to do to enable it. DS 'just does it'
3) DS itself requires almost nothing itself. Without you writting anyhing down the channels, i don't belive it does anything after initial log on. After that, it's up to you. The larger your network messages become, the less the overhead is, but the more the bandwidth need.
It's no different to custom code or even raw TCP/UDP in that respect. The overhead is measured in bytes per message, not kbytes, but I couldn't tell you how many, but my guess from what I've seen is in the order of 10-20.
If you thrash the system with messages of around 20 bytes of your own game data, then you will be wasting 50% of your bandwidth, but if you bunch them up to a few hundred bytes or a couple of k, then the overhead becomes alot less significant, exactly like tcp.