Just do what most people do. Heat the plate gently to reshape if needed and glue it on. Ask anyone who has ever trooped with me. I test my suits to the limits and the glue I use has never let me down.
Its amazing how much more detail is present on the pvc lids compared to the abs - yours looks amazing so far.... if i was you i would leave the tube stripes off....
Hand painting your helmet details alwasy looks better, using the template is best if your not confident to do them free hand, the hand painted decals look very good too, if your hopeless and still want the accurate look..