1 - Full parry is still an offensive maneuver. You can either use the 2 sai in full parry and use the OB and +10 as a defensive bonus - or - you can use the Sai as shield weapons and get a bonus of +30 (+15 per weapon). I suspect you get better results from the full parry application.
2 - +15 bonus only applies if the weapon is used equal to a shield = no attack, no parry, etc.
3 - Shield Training is required for the +15 shield benefit.