Mantey's plan for an American attack has been well known for YEARS. I've read "The Weltpolitik of Wilhelm II and the United States" in the Library of Congress about 4-5 years ago, which has all the details of Mantey's plan.
I'm a wargamer, have played around with setting this up as a naval scenario. The naval forces are actually quite well balanced in terms of pre-dreadnoughts in the 1900-1905 period. (Americans a pretty short on cruisers and scouting forces, because Congress only budgeted for the big ships.) But the Americans have the advantage of defending. Interesting contrast in ships as the Germans went with smaller, quick-firing guns on their pre-dreadnoughts while the Americans had larger guns with a slow rate of fire.
If you read the various quotes and statements, there actually was quite a bit of ill-feeling on both sides towards each other....a German fleet had hung around the Phillipines hoping to pick up scraps from the Spanish during the Spanish-American war, and had been directly threatened by Dewey, and the Germans had wanted Samoa too.
I have little doubt the land invasion would have failed miserably eventually, as they were overwhelmed by hordes of enthusiastic and fired-up, if ill-trained, volunteers and draftees.
As others have noted, having a "plan" for something isn't that big a deal, though to non-military media types it seems like it is. The current US military, I'm sure, has a "plan" for invading every single country in the world. And the fact that this plan was done by a lowly Lt. should tell you something. It was a BIT more serious than an idle War College excersise, but FAR less serious than a plan developed by Admirals and Generals.