The special deal on pricing at the moment is too good to resist!
Is the 6 months license validity from the day you pay or the day you register the license on theme.co?
ie. If I wanted to purchase an extra license/s for a future project (and then register it when ready to start developing), is it worthwhile doing it now without losing the 6 months of auto-updates?
Automatic updates for the theme and the bundled plugins will always be available as long as the theme is validated. There is no expiration of the licenses that you purchased.