However, the concept of the Autobahn is not exactly splitting the atom. The development of automobile infrastruture could conceivable be either faster or slower, but in the grand scheme of things, I don't see any modern state being shackled to rail forever. I always understood the USA led the automobile industry with the introduction of mass-production and the Germans were merely high quality contributors - I don't see that changing too much.
The CONCEPT of the Autobahn is simple indeed, and there were precursors in other countries (in the US, or more specifically: in the state of New York; in Italy, in Canada, and in Germany itself between Köln and Bonn). Note, though, that all these were short limited access toll roads in and near highly populated areas, whose control of access (no turning onto the road at all points) was primarily meant, besides allowing toll collection, to allow drivers to enjoy the joy of high speed and to reduce pedestrian / non-automobile traffic participant injuries and casualties, the numbers of which (while really low when compared to today) were still (rightly, imho) considered a great scandal back then, while today we've just learned to accept them. The historical US term "parkway" illustrates their purpose and character pretty well.
Turning this concept into a nation- and continent-spanning spiderweb of massive and expensive concrete infrastructure (complete with tunnels, high bridges etc.) designed to allow fast transportation of goods and people (and military material) is, again, something which many people could have come up with. But I doubt that it would have been tried and implemented in a democracy first. There had, actually, been Autobahn plans being thrown around during Weimar Republic times, but they had been considered a crazy idea, given the frail public finances and the massive amount of property confiscations etc. this would have required in Germany.
Note that the large US interstate system is basically an heritage of Eisenhower's presidency, i.e. it was implemented only after the Nazis had done it first.
It is no coincidence that the transformative leap from petite private parkways to massive omnipresent freeways was done in an extremely totalitarian dictatorship which was building up for war. (The Führer had tanks, not Volkswagens, in mind to roll over the Autobahnen, and so they did.)
You said, rightly, that the US was leading in automobile mass production, and no other country had joined on that bandwagon yet. I agree that German automobile industry may have continued in the luxury section (much like the British, too), for that is what automobiles were in the 1930s, and what they may have remained post-WW2 in a world without omnipresent freeways (and good quality rural roads to go with them, too): a luxury good. In a country with such vast distances such as the US, and such a weakly developed public transportation system, the switch to mass individual automobile transportation may have been logical or even inevitable at some point, even without freeways, or at least without so many of them. But in the smaller, much more densely populated countries of Europe, at least where railroad infrastructure was extremely extensive, this may never have become a reality. Even more so, if we consider that without the two world wars, the role of the world's greatest power and the no. 1 source of cultural and political inspiration for Germany might not have shifted from Britain to the US. And in Britain, both trends (automobile manufacturing being a luxury toy, and railroads being good and everyhwere) were even more pronounced than in Germany pre-WW2.
I understood the Anglo-Saxon obsession with home ownership was still not a German priority, or am I mistaken?
It wasn't in 1914, and it even still wasn't so much in 1939, although things had begun to move slowly in that direction, both under the Republic and under the Nazis. But ever since the 1950s and the "Wiederaufbau", in which millions of small houses in suburbia and quasi-suburbia were built (where people had lived in large "Mietskasernen" or in the tiny houses of crammed old towns before, but WW2 had reduced much of that to rubble), the obsession with home ownership is a thing in Germany, too. Of those who graduated from school with me in 1999, I'm almost the only one who hasn't bought a house or at least flat of their own to live in (and even that only because, as a scholar without tenure yet, I'm still moving around the country a lot).
Whatever the scientific environment was in Germany up to 1914, it outperformed its contemporaries.
No doubt about that.
I would expect state funding to be more dependable than private funding if there is a prolonged economic depression
Hm, not really. In the crises of the 20th and 21st centuries which were caused by market collapses, the private sector crashed hard, governments struggled to do something about it, the private sector recovered relatively speedily, but the government is stuck with staggering piles of debt which greatly reduce their space for maneuvre. Now, a Germany in the boundaries of 1914 may be so relatively autarkic as to afford implementing hair-cuts on its own public debts, so maybe things wouldn't be THAT bad. It was just one tangent into which the development of German research could have gone which would have derailed it from the shining glorious future some people painted for it. That_could_have happened, but Germany could just as well have taken the path of Britain's institutions of higher learning, which don't have a great 20th and 21st century of applied scientific breakthroughs. The anti-natural sciences, anti-applied sciences attitude was certainly there among German intellectual circles (Dilthey's invectives against the natural sciences comes to mind, Weber's anti-scientistic approach to sociology...)
The Junkers were influencial, but the socialists were gaining influence - lower tariffs would reduce the price of food for the masses and force the Junkers to improve productivity in agricultural production.
I doubt an SPD government would focus on lowering tariffs, unless it had to form a coalition with the Progressives or even National Liberals. If they get really strong, like through a Russia-in-October-like revolution, they'll nationalise industries and divide Junker land among the peasantry. If they lead a parliamentary reform coalition, then the Catholic Zentrum is an indispensable partner for any centre-left alliance, and their focus was on South Germany's small-holding peasantry and crafters, who weren't overly eager about free trade, either.