The thing is: the Germans will never be able to keep the Americans out of the war if they rely on submarine warfare. And every German attempt to sow dissent in the US would only bring the Americans sooner to Europe, not later.
Best way round that is probably to bring the Russian Revolution forward a few months. Iirc there was a general stike in Petrograd in Oct 1916, and troops sent to crush it opened fire on the police instead. In the end, the Cossacks restored order, but had they been unable to, Petrograd fell into rebel hands, and things spread, then Nicholas is probably gone by November.
In that situation, Bethmann and other opponents of USW have a far stronger case. Why acquire a new enemy just as an old one is (or may be) dropping out? So USW is at least delayed, and maybe avoided altogether. This, BTW, does not prevent a big increase in sinkings, which were going up even before USW. Had sinkings continued at the level of Jan 1917 (the last month before USW) there would still have been more sinkings in 1917 than in the previous three years put together.
Actually, though, I wonder if, with a bit more finesse, the Germans might even have gotten away with USW. Rodney Carlisle's article at
http://cnrs-scrn.org/northern_marine...17_3_41-66.pdf
shows that several American ships were sunk by u-boats
without generating a response from President Wilson. Not until the
Vigilancia was attacked without warning on March 18, and fifteen Americans killed, did he decide that Germany had committed an "overt act" of war against the US.
And interestingly, according to the site at
http://www.usmm.org/ww1merchant.html
such attacks weren't all that common. There were a couple more, the
Healdton and
Aztec, over the next few days, but thereafter none of comparable seriousness until April 28, three weeks
after the declaration of war . The majority of attacks on US ships were by "cruiser rules". This suggests to me that had the Germans discreetly instructed their subs to refrain from USW-type attacks on American vessels (without publicly acknowleging that they were doing so [1]) they still had a chance of avoiding war with the US. Even now, Wilson wasn't eager for it.
Even if this resulted in one or two merchantmen per month getting away, it would surely have been worth it to preserve US neutrality [2] - but this was evidently too subtle for the Kaiser.
[1] Indeed, in
The Victory At Sea, Admiral Sims expressed the opinion that Germany
was doing precisely that, to facilitate an early peace with the US. Sims was apparently mistaken (the u-boats probably just did it that way to conserve torpedoes) but it's significant that a man in his position could have thought such a thing. Had he been right, of course, it would have been a classic case of shutting a stable door after the horse had bolted.
[2] Especially as the unused torpedoes would presumably not be wasted, but employed against a ship of some other flag. So total sinkings would probably not have gone down.