So, Summarizing the answers would be:
1) Only one special materiel is allowed, and the Armored Coat counts as metal for available crafting
2a) When layering special materiel armor, only the most voluminous armor counts as per piecemeal armor rules, I.E. biggest, heaviest piece is applied. Best AC bonus counts, all other penalties largest counts.
2b) When layering armor, the outermost layer enchantment effect applies and the AC bonus only applies to the enchanted AC value.
Does that sound about right?