If we put multiple pumps in parallel every 250 segments, it seems like we should still be able to effectively get unlimited flow over long distances.

For example:

One pipeline splits to allow 10 pumps to be connected in parallel. The pump outlets are then merged back into to a single pipeline ...

In terms of killing these enemies, was anyone else's first thought sacrificial nuclear reactors?

Build and fuel reactors in enemies path.
Wait.
Enjoy the fireworks.

The resulting nuclear explosions will hit many segments of the demolisher's body and should do enormous damage.