I might release it soon, but now I'm trying to understand how characters become invisible (aka disappear) in animations.

I have private messaged ya, reply me there. I'll try to help you with that.

You can edit textures, they're basically flipped BMPs in WORLD.WDB

I made a script for Unity to convert LEGO Island's ANI into Unity's ANIM in Unity Runtime, works almost perfect. Requires Actors to exist in Resources/Actors folder.
I don't know if I should upload it somewhere in public right now, as it's unfinished

I managed to replace some textures in game, they are so simple. It's basic BMP 8 BIT, but flipped.