There isn't any way to hide the UI in this old demo version, though there is a keybind to do so in the planned upcoming version.
1) You can skip the dialogue if you want to get to the gameplay, or do fast forward (CTRL), although that isn't mentioned in the demo.
2) I think the current behavior is fine.
3) The lock-on feature is supposed to address this issue, although it isn't in this demo.
4) The guard counter and speed roll timing have been adjusted in the upcoming version already, which makes it easier to perform and have a brief moment of invulnerability.
5) If you hit the ranged goblins, their abilities get cancelled immediately.
6) There will be a heal spell, but it isn't in this demo but u can use potions in this demo build.
Thanks for the replies! Concerning the ranged goblins, when there are four of them spread out, spewing poison breath and projectiles (and many other enemies attacking), hitting to interrupt isn't practical. I guess one possibility I haven't tried is to lure some of the enemies away from the group, to fight a smaller subgroup.