I wonder if an Agadir-sparked war might have had more of a naval component than did the actual conflict IOTL? That is, might there have been battles in the Mediterranean, near the site of the original flash point?
I also have to wonder about the presence/absence of Italy (somehow, I have difficulty in believing Italy would join its Alliance partners over a the Agadir crisis), and whether Sweden might have jumped and joined the Central Powers, as they almost did IOTL.
As to the US: at first, I don't believe there would have been much sentiment favoring involvement. At that point, the US had no stake in the proceedings. Perhaps Taft might have offered his own services as an arbitrator (who better, one might ask, than a judge?), or perhaps offered the services of the US in general, with, say, Elihu Root as the chief mediator (by then, the Taft/TR split had probably deepened sufficiently that TR wouldn't have had a chance at an encore to his '06 Nobel Peace Prize).