Wasn't that because everyone knew the war was lost by the time the US joined the war?
Years of blockade took its toll, and there were seemingly endless numbers of fresh American troops arriving in France to complete their training. Germany did make one final push on the Western Front that almost succeeded, if it weren't for under-trained American being rushed to the front line to stop the advance.
On the first part, not really. Yes the Americans helped, but during the german spring offensive it was more in bolstering Morale in France and Britain and with equipment and supplies. They then proceded to make the same mistakes everyone else had learned not to do in the previous three years of slaughter, when they arrived. The german Spring offensive didn't encounter all that many American troops, it was explicitly started before those could arrive, it mainly failed because it had no real plan and strategy behind it. They just rushed everyone forward through terrain the germans themselves had destroyed when retreating to the Hindenburg line a year earlier.Wasn't Germany also contemplating taking over Austria-Hungary because of how incompetent their military had been throughout the war?
What was worse was because everyone in the Austro-Hungarian Empire had different cultures and spoke different languages, the same orders had to be translated into several different languages before things could get done. It was a nightmare coordinating every unit in the same army to be on the same page for any campaign.
Instead of aiming for strategic targets, like splitting the British from the French and threatening their supply life-line in Amiens, they just took as much land as they could. Pointless waste of men.
It also destroyed morale, because the german troops found out how well supplied with food their allied opponents still were.
Once the Entente started the big Offensives that ended the war, german troops would just surrender in droves.
Austria-Hungary's military had been a german puppet in all but name for years at this point, completely unable to survive on its own, after the Russians crushed them. Not sure if it was during the Brusilov-offensive or even earlier. That's not to say their soldiers couldn't fight well, but as you said too many languages, bad infrastructure, even worse leadership (looking at you Conrad Hotzendorff) and of course many of the soldiers from minorities (aka neither Austro-german, nor Hungarian) had no real incentive at keeping this state alive. (again that is not to say that none of them did, one of the best Austrian generals was actually a Serbo-Croat)