The platform is too high to reach, so you can get stuck but the Slime Girl is the end of the demo anyway. Likewise, it's possible to skip the scene with Rosaline and instead head down to the room with the two Damsels where you'll get stuck since the Damsel doesn't spawn and you can't open the door to confront the Slime Girl. I believe there is an option in the pause menu where Vivi can be tied up and respawn at the last save point.
As for the door issue, I'm guessing
Boundforgreatness intended you to touch the orb at the save point where Vivi gets tied up and meets Rosaline and then she would head down after escaping. It could be an oversight in her part if she didn't expect players to skip the scene and head straight down where the end of the demo is at.