No, you don't have to have contact resistance applied everywhere. The contact resistance is there to represent something that isn't modeled in or represented by the fluid flow. Like a layer of paint, or an air gap within the model while "exclude cavities without flow conditions" is checked.
You could try another finer local initial mesh on that resin layer. I'm sure that will be computationally intensive. That is your only option if you do not simplify it using contact resistance. Can you create split faces on one model and apply different resin thicknesses via contact resistance that way?
Sure, you can absolutely apply a contact resistance instead of meshing the air gap.
Like I said a minute ago, contact resistance is like representing a layer of paint. If the air was paint, you would only apply the contact resistance to one part. If you applied it to both you would double the resistance accidentally.
I am not sure of a database with resistances, sorry. You could convert thermal conductivity into thermal resistance using R = t/k, where t is thickness and k is thermal conductivity.
I have confused myself by what you mean by multi-layered solid components, so I don't have a tip for you.