Hello!
Regarding the depth change, this is because I'm using depth = -y for depth sorting, and since the game is still 2D, the light is just a "plane," and they're organized into layers, like other things. You can simulate smooth transition by applying a shader that darkens the tree depending on the light's position (but this isn't included).
I need to make it clear that it's more difficult to deal with depth in 2D if you're trying to imitate 3D perspective, like in top-down Zelda-like games, but it's possible to get good results.
Yes, you can change litType to LIT_ALWAYS. This will make the light always stay on regardless of the depth. You could also experiment "normal maps".
What graphics card do you have? AMD or NVidia? I can try to fix that.
I understand the price issue. I sometimes offer discounts, so you can follow me to be notified. You can get a refund if Crystal does not deliver what was described.
I guarantee you won't regret getting Crystal, it's currently the most complete library for GameMaker, and you have complete freedom with it.