Memes about GAINAX running out of money to finish series go all the way back to Gunbuster, which was their fourth production, and second under the name GAINAX.
The second production, Daikon IV, survives thanks to piracy because they didn't license them music used :V
Unfortunately I suspect that outside of the people directly involved we won't ever hear properly. The rumours flew around since early 1990s, and there is the part that Gunbuster was originally supposed to be a TV series (AFAIK), and as result the plot was extremely compressed into those 6 episodes that we got ultimately.
And it's not like the studio was not known otherwise for wild swings even when they didn't run out of money... (anyone remembers how Mahoromatic ended?)
With B&W you need to transfer the shading and you can't just (literally) fill the gaps with a colour.
The second half as a whole is clearly made on the cheap, but episode 19 is special, it is all paper cutouts on a stick, most likely from the storyboard, on top of recycled backgrounds.