-Wilhelm II dies in 1900-1901, Crown Prince William takes over as Wilhelm III
-German policies favor less militaristic expansionism in favor of more commercial methods. Also colonial policies are made very light-handed with large autonomy granted to the colonies
Fairly plausible, but with a Prussian-unified/dominated Germany, it's always going to be pretty militaristic.
-Austria-Hungary collapses into civil war in the early 1920s with Austria (modern Austria + South Tyrol + modern Czech Republic + Gallicia) voting to become part of the German Empire. Hungary, Greater Serbia, and Slovenia-Croatia see heavy German influence in economy and education.
I would think that the Germans would have to choose Slovenia-Croatia or Hungary. Since Croatia was part of the Kingdom of Hungary legally, the Hungarians will inevitably have irredentist claims on Croatia. I assume Slovakia is part of Hungary. What would be the status of Transylvania?
-Wilhelm III's family marries into Luxembourg which results in its joining Germany, albeit as not the most patriotic of provinces, and results in the last western expansion of Germany
I don't have any problem with this, but dunno if they'd actually annex Luxembourg.
-Russian Empire collapses into civil war in the 1930s as monarchists battle socialists, monarchists win only by brute force but Finland, Poland, and the Ukraine are liberated as the price of aid from Germany. Ukraine is actually a neutral country, the former two quickly become German allies.
I'm pretty sure Ukraine would be the main focus of German investment etc, and Sevastopol would probably be a naval base for the Germans. And I'm not 100% sure they'd support an independent Poland, with the nationalistic issues the Germans had in Posen, Danzig etc.
-France, Germany, and Italy sign a treaty permitting reduction in border tariffs and creating a European Coal Commission as an economic counterbalance to the massive UK economy in 1950. UK responds by bringing the United States into the Commonwealth's economic system without joining the Commonwealth itself
Why would the USA want to join that system? And there'd be massive repercussions on the Dominions, most of them negative for the Dominions, and for the UK itself.
-Wilhelm III dies in 1954 and is succeeded by Emperor Louis I, whose wife Juliana is queen of the Netherlands. Though the union scares many who feel it will bring the two nations together, this is specifically forbidden by the pre-marital contract, with the first son taking over Germany and the first daughter or next son taking over the Netherlands.
-German assistance to China allows its unification to occur albeit later than OTL and industrialization to proceed rapidly, eventually eclipsing Japan as the primary native power in Asia
Plausible, but I don't think the Russians, or even the British would be happy about this.
-Decolonization only mildly affects German interests as two of her colonies, Tanzania and Kamerun, remaining part of the country. Namibia leaves of her own accord in a peaceful way while Togo is racked with internacine warfare almost from her independence day
Being the most sparsely populated, I think Namibia is the most likely to stay, with Tanzania being the most likely to gain independence...
-With the start of the Space Race in the late 1970s, Germany gains international prestige by becoming the first nation to land on the moon in 1989.
At least this has been put back a bit, but without the Third Reich, rocketry technology is going to be very far behind.
-By 2000 Germany dominates continental Europe politically and German is one of the five most widely-spoken trade languages along with one of the three most spoken scientific languages. Her (OTL late 1970s/early 1980s) technology is cutting edge around the world, for while the United States and China maybe the factories of the world it is Germany, and to a lesser extent the UK or Japan, that serves as its laboratories.