Shit, dude, I clicked on this thread thinking "whatever, I'll look at this new WIP, probably nothing fantastic though" and was immediately FLOORED by the title screen art. It's freaking gorgeous. Anyway, your questions:
1. The "main character with amnesia" thing is used pretty often, but the bit with the maze and the demon is interesting, so it seems like you've got your own spin on that trope which will make your VN interesting and worthwhile.
2. Isaac. He's cute, for one thing (I know, I'm shallow, shut up). That conflict of the happy memories but sad feelings also has me interested, though, and in these amnesia-type stories I'm always interested in the moments where the main character finally remembers something from the past.
3. YES, that's amazing. It's pretty much inevitable that some endings might have more clearly positive or negative elements, but sometimes in VNs when a developer sets out to create specifically good and bad ends, they just feel too over-the-top. (Bad end: You didn't get the girl, her dog died because of that, your dad has cancer now, and as you were walking out of the clinic after the diagnosis you got hit by an ice cream truck. Good end: You got the girl AND your promotion, the girl's controlling father had a change of heart, the evil corporation boss who was barely relevant has been defeated, and your mom's been alive the whole time. Like... okay, I guess, but that's quite a few coincidences.) Endings with good and bad elements are more realistic and more interesting.
4. Yup! It seems like a cool project! ^_^