I'm planning to add artifacts, prestige upgrades and even normal upgrades in the Spelunker's Lodge building, that will increase Mithril drop chance. I'm even thinking about adding occasional active "golden cookie" clickable events that can drop mithril.
It's unlikely you'll be able to transfers saves from the demo to the full game, because a lot of new stuff will be added and restructured, which would be difficult to keep properly functioning and not breaking. But I think the joy of these games is anyway to replay them in fresh new light, and there will be bunch of new content to try, so I'd think as a player I'd want a clean new slate anyway.
Thanks a lot for playing, wishlisting and the feedback!