A) Specular shouldn’t need to change, UE4 uses metallic workslow, and the default value applies to most materials, look at the mud example with water here Tutorials & Resources | Marmoset
B) I haven’t see it officially listed anywhere, but I’ve seen some forum posts suggesting accurate sky and sun values, like PBR lighting questions - Rendering - Unreal Engine Forums
C) I don’t think you need to modify the specular looking at the marmoset post from A)
D) I would just multiply the basecolor with itself, since wet materials are typically darker and more saturated, you might have to compensate a bit to stop it from becoming over saturated.
You may want to use the Clear Coat shader if you want to simulate a surface so saturated with water that it’s starting to puddle.