Please refer to the documentation to get information on how to create a custom trail shader and how to display its properties on the Magic Trails Panel. As a general tip, use the UV Map node as a basis for your effect and build the material around it.

You can instance the components of magic trails using GN but they will NOT be calculated for every instance individually. Unique simulation is only possible for individual trail components.

Yes but with certain limitations. Trails need to be baked. After baking, a hidden armature is being generated, controlling the motion of the trail mesh. Use it when exporting. All the preset trail materials are blender-specific, meaning they would need to be rereated in-engine or replaced with a compatible or custom shader.