Thanks for the diagram. It does not have any loops in it - links, yes, but not loops.
Having seen the diagram I'm not sure I understand what you mean (in your Original Post) by this
voltage of around 2.5V appear on all digital/analog PINS of all the connected Arduino's pulling to the level HIGH of PINS.
My wild guess is that the way you have everything connected is causing the Arduinos to reset over and over again at high frequency. And I don't know how you would avoid that. Maybe you need diodes between the reset pins and the common connecting wire so that they don't interfere with each other.