Thank you for trying out the demo! I'm glad you liked it so far :)
And thanks for the report, that is indeed a bug! There's one other main quest after that, a side quest and some feed quests. I've fixed it on my end now so I'll post a fixed build soon :)
I'll look into the loading issue some more because it shouldn't be related to this quest issue (unless it only happens at this one point in the game and you were able to load saves elsewhere?); one thing I can think of since you're on Mac is that maybe you need to enable read/write permissions? The game needs to write to a file to save, then read from it to load. If that's enabled already it would be an issue on the game's part, so I'll look into things!