Why were the Yellow River dyke repairs delayed?

The wiki articles' focus on the Communist/Nationalist dispute create an impression that repairs were delayed due to political disagreements. However, while certainly a factor, the most significant source of delay was mundane and technical: the repair works were destroyed by floods.

Efforts to seal the breach began in March 1946, with an original goal to finish within half a year. The gap was finally closed only in March 1947, making for a six month overrun. Although this process was punctuated with intense Communist/Nationalist power struggles, mother nature played a much more decisive hand:

The building of the dams was completed in June, 1946, only to be swept away by the summer flood in August. The second time the dams were built, the same disaster occurred, making half a year's hard work futile and $28 billion a total waste.

_{"The Yellow River Returns to its Old Course", Water and Water Engineering 50.1 (1947): 314.}

It's telling that the works were finally completed over winter, when water levels were as little as a quarter of their summer peak. While it would be total bonkers to take either party's official lines at face value during this period, I believe the technical specifics cited by the latter in an announcement is credible:

According to the government's publicity department spokesperson, Peng Xuepei [in winter] the water level was not too high and the flow capacity was only about 1000 meters [sic] per second . . . the high-water periods that began in late March, when the flow capacity would be 4000 meters [sic] per second, and peaked in late July, when the flow capacity could be 30,000 meters [sic] per second.

_{Edgerton-Tarpley, Kathryn Jean. "From “Nourish the People” to “Sacrifice for the Nation”: Changing Responses to Disaster in Late Imperial and Modern China." The Journal of Asian Studies 73.2 (2014): 447-469.}

Of course, technical setbacks were not the only reason for the delays. There were two dimensions to the political troubles plaguing the project.

Firstly, after 1938, the old riverbed of the Yellow River became 800,000 acres of fertile farms worked by a population of 400,000. If the Yellow River were to be restored to its pre-war course, it would necessarily and literally wipe out these farms. While the people could be evacuated, this was a costly and complicated process in and of itself, and their homes and crops would be destroyed. As it happens, this area was under Communist control in 1946.

Secondly, as fighting resumed during the repairs, a military dimension emerged. As the saying goes, "amateurs think strategy, generals think logistics". The fighting at the time was concentrated between the new and old courses of the Yellow River. If the Yellow River were to returned to its original riverbed, it would cut Communists forces off from their supply bases.

As UNRRA personnel recalled, the river had once again "assumed strategic significance." Re-diverting the river would benefit the Nationalists by severing links between CCP forces and their supply sources to the north and west of its pre-1938 course.

_{Muscolino, Micah S. The Ecology of War in China: Henan Province, the Yellow River, and Beyond, 1938–1950. Cambridge University Press, 2014.}

It was thus in Communist interests to delay the repairs for as long as possible, and vice versa for the Nationalists. Since they can't very well argue against the project itself (described by chief adviser Oliver Todd as "an opportunity to increase the world's food supply by an estimated two million tons annually through the rehabilitation of nearly two million acres of good farmland"), Communist PR focused instead on advocating for protection and compensation for the riverbed farmers.

Both of these angles of Communist consideration can be gleamed from their official statements, which are referenced in the question. Nonetheless, we ought not overestimate the relative impact of these disputes. In fact, constructions appeared to have progressed very quickly even under military fire, which was the epitome of a political conflict:

The work went ahead quite rapidly, even, towards the end, when the workers came under shelling from the north bank of the river . . . the ceremonies gives no indication that the area was about to fall to the CCP and that the GMD and UNRRA had, in effect, performed a major service to the CCP.

_{Lary, Diana. The Chinese People at War: Human Suffering and Social Transformation, 1937-1945. Cambridge University Press, 2010.}