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.
Sola lingua bona est lingua mortua
ἡ μόνη ἀγαθὴ γλῶσσα γλῶσσα νεκρὰ ἐστιν
lišanu ēdēnitu damqitu lišanu mītu
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