Great demo, very fun and addicting! (I spent a little too much time playing it when I had other things to do). Although some of the mechanics are not so obvious at first, once I understood them all I felt like I was really in control, and appreciated every mechanic.
I definitely second everything said in the other two comments, but I have a couple things to add as well.
Alchemy:
One of the biggest issues I had was with the fact that the alchemy requires exact levels of the ingredients, combined with there being no way to actually undo adding an ingredient (there's the minus, but that uses more rather than giving back). Early on, I had thought the levels didn't need to be exact, due to the fact that for the regular blue or red potions, it completely ignores the other color. This led to me, in later game, adding a bunch of ingredients to make the level 5 potion, only to realize I didn't have enough red/blue mushrooms to make it exact. To make matters worse, since there no "clear" option, I had to use even more ingredients to bring the levels back down after.
I get that this is intentional to require exact amounts, but perhaps either make it exact for all or none of them, or at least allow an undo option in case it's misunderstood. As Kamoo suggested, having a tutorial here would help too.
One other thing I was thinking was that it felt like I didn't have a lot of freedom to experiment with different "deck" combinations, since everything is only ever upgrading the cards already there. That's one aspect I really love about deck building games.
Progression:
I understand that the progression is intended to make it more challenging and avoid grinding. However, there are a couple issues I had with it.
First, the fact that you can't replay a level is a little frustrating. This can easily make it such that if you make mistakes early on, these compound later, making it very difficult to catch up. It's not very friendly to people who take a bit to get used to the mechanics.
Second, per my above frustration, the mushrooms from early on are essentially required for the later potion levels to get them to match exactly. Since you only are pushed deeper, and don't find the mushrooms later, you will be screwed over if you didn't save some mushrooms early on.
Other Minor Things:
After completing the demo, I couldn't help but feel like I could have gotten a better outcome had I made it deeper into the last section, Though there was no obvious indication if this were the case, so perhaps there could be an indication there was nothing you could have done to change that outcome, or if there was invite you to try again?
Last minor thing, which you'd probably update for the final release anyway: It would be good if the characters had different images for different expressions depending on the mood. Since this is a demo though, I totally understand it not being fully polished with things like that. One thing in particular I noticed was at one point Twig said Lizard was in her cloak, but in the image he was still sitting on her staff.