Good morning: Thermal couplings are well documented in the on-line documentation; please check there. Also, we at Maya provide training on a regular basis; please contact us if interested (www.mayahtt.com). For brief answers to your questions:
1.) Nodes and elements are never modified with thermal couplings. TMG defines finite-volume calculation points at element centroids. When the "perfect contact" option is used, the associated calculation points are merged.
2.) The mesh is never modified.
3.) In general use the smaller of the 2 surfaces as "primary" and keep relative element sizes similar. More details are covered in both the documentation and training materials.
4.) The primary element face is subdivided for resolution purposes when establishing thermal couplings; the factor controls the number of subdivisions.
5.) The most common reason for thermal couplings to fail is lack of underlying elements when defined on geometric entities.