Interesting. Check BDAG. It cites Matt 2:10 as an example of use with an adjective, rather than with a verb. And the KJV supports that use too. Now the NASB seems to support Wallace's view. What's odd given his association with the NET bible is what it says : "Matt. 2:10 When they saw the star they shouted joyfully.15" And in particular note 15 is interesting saying this "Matthew 2:10
15 tn Grk “they rejoiced with very great joy.”"
So I don't think its ambiguous at all
And as to the syntax diagram, I'm no expert but I read it the way you describe. But Marco would have to comment on whether that was the intent or not.
I had a quick look in Wallace's grammar. I don't think his description of the cognate accusative is directed at σφόδρα but at χαρὰν and his text could have rendered it : "they rejoiced with exceedingly great joy" without damage to the cognate accusative. Or at least so I would think though I have only enough Greek to be dangerous and mostly so to myself. Actually looking back on my mail this is basically what note 15 in NET says.
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