Unfortunately there is no way to persist time on the mcDemo205 across a reset at the moment. We are working on improving this but the problem is trickier than it might appear on the surface.

As for your questions:
1. There is no such thing as a sigfox timestamp. All timing information is handled by the base station.
2. There are no plans at the moment to support sigfox downlink on the mcDemo205. The device is only certified for uplink by sigfox.

Another update coming soon is the ability to set the device time which on an mcDemo205 can be retrieved from the on-board GPS if a fix can be acquired.