The Millionaire's Unit by Marc Wortman provides a great narrative of the first American Airmen in WW1.

While they were "volunteers" these guys were mostly Ivy league-ers (from the Yale flying club) whose parents were some of the most influential and wealthy people in America; literally the Rockefellers. An exception to this was a celebrity race car driver whose name I can't recall. Their involvement was highly publicized at a time when Americans were very much in favor of the Entente Forces. To the best of my knowledge, there's no stories of American airmen volunteering for the Central Powers, mostly because there weren't that many Americans who knew how to fly and the ones who did were rich and patriotic.

These volunteers weren't just random guys who "joined up" and the French threw planes at them. Airplanes and flying boats in America at the time were a luxury/toy of the rich. The men who volunteered in France were the only people who really could volunteer, because not many Americans had any experience flying and even fewer could afford a plane. Offhand I know Lt. David Ingalls, and I'd guess a handful of others, at some point flew for the British as well.

Further, there was a spirit amongst wealthy Americans (back then at least) that military service was expected of the rich. The wealthiest felt it was their duty to send their kids to serve the country which had provided them such ample opportunities. Basically, the motivations of these men were mostly adventure and patriotism, not really to be "mercenaries". If a wealthy family sent their support to Germany, it would most likely have been treated by the press like if one of Bill Gate's sons declared jihad and went to fight for Al Qaeda.

TL;DR: There were very few Americans who could fly and volunteer to fight in the war, and they were almost all well-known Yale/Harvard educated children of the wealthiest families in America. When Americans were very much in favor of supporting the Entente, an American millionaire's son flying for the Germans would have been publicized and most likely heavily derided as a traitor.

The Millionaire's Unit is a great book for what you're interested in, supported with primary sources and it reads like a movie script. Would highly recommend.

Most certainly! The RFC had around 200 Americans serving during WWI, including a number of aces. Wikipedia has a list of the aces, which is a nice list. A lot of them transferred to the US air corps after US entry to the war.

I don't think there were any who joined the German forces, given substantial anti-German sentiment. Eddie Rickenbacker, for instance, changed the spelling of his name to be less Germanic near the outbreak and later became America's leading ace.