You have good taste in games! Have you played Alice before or is this your first time? I think this is a really neat idea, but you might get even more ideas when you finish the game (if this is your first time) so watch out
I'm not sure about the proportions of the 5g you plan on getting, but my vote is for a long and shallow tank, I think it would be easier to make it look like the game. Have you thought about using black sand as substrate?
I agree with the suggestion about glass teacups, unless you can somehow verify that normal ones wouldn't leach anything into the water. I would also say that kid's teacups are probably even less safe than normal ones, because they aren't specifically designed to be used and go through the dishwasher.
One note about the blue tiger shrimp, I've heard they're really expensive and delicate, just something to keep in mind.
I'm really excited to see the results of this, especially the mushrooms!