I'm glad you're enjoying the game!
Obsidian tools are weaker than bronze, but they never expire (so it's kind of a tradeoff). Obsidian is also much more common than copper (once you have a pick strong enough to mine it), so that's part of the reason it's weaker.
Greater creatures have a 5% chance to spawn in no-star biomes (which is all that exists in the demo), but there's a guaranteed one of each type in arenas.
Old saves will be compatible with future versions. Areas you've explored won't re-generate, so any changes to those areas won't transfer over to old saves (for example, the goxen arena wont spawn in saves from before I added it), but the majority of the world will spawn around the demo area, so you won't miss out on much.