Read mode does not change the display, but init blank acts the same as init visible. I'm guessing the read circuitry is still on the chip but the data out pin isn't bonded out. I don't know why init blank doesn't work.
I set segments in row 0, then just raised /CS after the row and mode data for rows 1 and 2, and rows 1 and 2 were blanked. It looks like the shift register is auto cleared.
Underclocking turns off the segments that were on previously but were not changed this time. Also explained if the shift register is auto cleared.