[Varyon]

Grabbing Parry points back to Hand Catch (MA82), which automatically grapples on a success. Additionally, if it called for an attack roll, with the opponent using a defense roll, there'd be no need for Rapid Retraction to cause a penalty to the initial Parry, as it would function just fine giving its normal bonus to defense. So, all told, I'd say the intent for Grabbing Parry is that success on it automatically starts a grapple, albeit a weak one (damage based on half ST).

As for how to handle the interaction with Rapid Retraction, the RAW looks to indicate that the character would have indeed failed their defense and been hit by the enemy. However, I think your handling of it - introducing a third state (Parried but didn't grapple, as opposed to the normal states of failing at both or succeeding at both) - makes more sense, and is arguably more fair. The RAW version has the benefit of simplicity.