Remote Update with NIOS and EPCS

Have you tried using the Remote Update Controller component instead of PIO and altremote_update?

I have never tried the PIO approach. Using the Remote Update Controller component, I have not had any problems reading or writing to the controller.

I have also never used the EPCS controller, so I probably can't be of much help there. My best guess would be an incorrect setting in your system library.

In the system where I got Remote Update working, I did not use the EPCS controller as it forces you to have a boot room in block RAM. As all my code was already stored in block RAM, this extra boot ROM was a waste. Instead, I made my own custom SPI component and wrote a few simple C functions to read and write the EPCS memory.