Don't forget hold person too.
Roll the attack and the crit. If the crit indicates a hold, target is caught in dragons hold. If crit does not indicate a hold, then make mnv roll to avoid being held. A crit PC gets +20, B crit PC gets +10, C crit PC gets +0, D crit PC gets -10 and E crit PC gets -20 to the roll. Difficulty will depend on circumstance.
Maybe; any attempt to parry a grapply should result in dragon getting a bonus to grapple mnv equal to ammount parried with (assuming parry is with a shield; my thoughs here are PC is placing himself in the way of the dragons attack, thus making grab easier).