I didn't envisage them holding off anyone, but rather chivvying along troops that were already running, so that more were captured, while the remainder had to run even faster and further than OTL. And the BEF couldn't fall back much further than OTL without losing Amiens and other crucial points.
That might be harder than you think. I've been reading several sources on the fighting during Michael and it seems that the British stood up and fought repeatedly in retreat, rather than just run away. The German infantry had to fight repeated hard battles as they went on and broke their foe again and again. There were some instances of British and French running away south of the Somme though, but that was generally when their ammunition was gone, in the case of the French not having arrived at all, and their artillery already having been overrun. When the artillery survived it held off the Germans with open sights firing and Lewis Guns.
I'm not sure the cavalry would have been able to exploit much before March 26th-28th when the way to Amiens lay open, but Ludendorff sent the 18th army south against the French.
Agree entirely about Ludendorff. He was so stressed out in Spring 1918 that I wonder if we can even call him sane. Of course, the US was an important factor there. He was racing against the clock, needing to get a decisive victory before American manpower made his situation hopeless. Without that pressure he may well have a much cooler head.
My read on Ludendorff was that he was in over his head. He was very emotional, even during Tannenberg having a breakdown, a reaction that was repeated throughout just about every tense moment in the war if Hoffman is to believed. Ludendorff was an over-promoted infantry colonel that made bad decisions when not supported by his ideas man, Max Hoffman. His whole 'chop a hole and the rest will follow, that is the way we did it in Russia' line smacks of amateurishness. Hindenburg was not interested in anything other than the lime-light, while Ludendorff had too much conflicting advice about what to do and had no concepts of his own other than to let tactical success decide the strategy.
Also his insane fixation on winning a complete victory let the opportunity to negotiate a peace slip by repeatedly, even after the US entered the war, because he would not let Belgium be independent after the war, thus losing everything in the end. I doubt a 'clear head', whatever that would have been for Ludendorff, would improve his decision making.