Yay! We've already found another way into Inbetween World! Let the searching begin again!

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So there is no key to open this door, once it gets blocked/locked?

I found another way out, and finished the demo, but I'm wondering how much I missed? extra XP wise?

You're not meant to get past this door. The level on the other side is the only way to open it, and you get to the lever via that hole that ends the demo...