1. I have no idea. Wondered that myself a couple of times

2. Any place with mines. Give priority to those inside your empire (Gabai, Propthasia(I think), Tarsos, Rkbaktana etc.), then Asia Minor (you should be well enough entrenched by the time Pontos decides to attack) then NE (Hekatompylos, Asaak, Nissa (when taken form parthians) etc.)

3. My standard anti-parthian armies (just killing them in my AS campaign atm, they're down to 1 city under siege) are amde up of : 4x pantodapoi phalagitai (cheap, AP secondary, good line holders), 4x persian archers (also cehap, they destory unarmored horse archers), 4 x eastern slingers(cheap, work wonders vs. parthian armored cavalry and I can replace them faster than PArthia can replace Dahae Nobles and Armored Horse Archers).