Just fired this up for the first time tonight. Restarted 3-4 times because when advancing Morticia through the 'signing up for class' path, I would always get a black screen if I tried the 'Ask why she looks so worried.' option. (Running 1600X900 windowed, if that's the reason why)
Anyway, the first restart from the beginning I did (forgot to save), I used Cheat Engine to push extra cash, because Rick let me work twice for some cash (40$ total) and then never offered again, and ended up buying everything in the shop on my first visit (also ended up going to the Sex Shop and getting Gwendolyn very early, but didn't follow up). I ended up getting Beth to offer the art class the next morning in the Living Room, before running through the first massage movie night (or maybe just after, since I didn't have a movie night fail for not having wine). Don't know what if any of this allowed the Art Class portion to work, but my second restart from the beginning never offered it up (or Rick's portal gun task for that matter, only offered plasma in the lava, so never could fly to Unity).
I suspect there's an optimal path through the progression of all characters combined, that can lock content if you fail to meet it.