1) don't know about seam tape for plywood roofing decks, but I would definately use the clips that join the edges together and to act as one piece. They are slid on one side and joined with the other piece. I use them about every foot or two.
2)for get the oil painting, go with builders tar paper for sure. Its your insurance against leaks. Condensation is addressed with soffett vents or eave vents.
3) Copper will naturally achieve that attractive green shad on its own.