RayTracing: custom shaders, reflection TAA off, per-material features, translucents are seen-through opaques

I am new to UE4 and I dived straight into rendering, shaders and RayTracing, and I have these questions:

#1 Can I write my own RayTracing shaders?

For example, I want to have different RayTraced Reflections color logic, where should I start? Should I modify FRayTracingPipelineStateInitializer? Are there examples somewhere? I know I can write my own shaders (never did that though yet), some Vertex+Pixel shaders avoiding Material Visual Graphs with Custom Expression nodes, so I guess I may tweak RayTracing too.

#2 Can I turn Temporal Anti-Aliasing for RT Reflections off to have always sharp reflections?

#3 Can I turn RT features off for different materials?

Like to disable RT Reflections for this one, RT Ambient Occlusion for this one and so on.

#4 Translucent (or Additive) objects are seen through Opaque geometry, which is rendered with RT Reflections (roughness doesn’t matter). See the GIF.

Is this known issue and is there a workaround to avoid this bug?