AHHH, thank you for your comment, Neci! They're always such a treat to read~
I'm actually super intrigued and happy hearing your experience regarding the MC. When writing her, we definitely wanted to highlight how her addiction is very much a problem and how she likes to stand up for herself and others could not only be a benefit, but also be a detriment in certain situations (i.e. meeting King Karamel).
In regards to Lakritz calling MC Sweet Tart because you're not the only one to wonder about that! It's just a nickname, and the reason why Lakritz choose it is explained briefly in his route, but it's a funny coincidence, right? ;)
To answer your question about the routes, the demo covers the entire common route. So, yes, you'll be locked into either Lakritz or Peppermint's route. You should be able to pick off right where you left off from the demo, though sometimes Ren'Py save files can be unloadable with updates and such. If that does happen, I'll include a choice at the start of the full release for demo players to jump into either Peppermint or Lakritz' route so y'all don't gotta replay through the common route if you don't want to!