By the way. You mentioned that you are struggling to understand shift registers. This article offers a good explanation: http://bildr.org/2011/02/74hc595/. This will also explain why the the complete state of the controller has to been reset each time a station is opened or closed. The shift register is cleared out and 8 new bits are sent to the register (or more if you are using an expander board; the linked article also explains how shift registers are daisy chained, which is exactly how the expansion boards work — I think).
So if the interval program is in the middle of a program (let’s say valve #1 is open), and you manually tell it to open valve #3 using the http command, does it automatically close all the valves first, then open valve #3?
I use the manual commands specifically to open multiple valves at the same time — so I can’t speak for the scheduler.