So if a pilot presses the priority button, the autopilot is disconnected but BOTH side-sticks have control? (algebraically summed together)
No! When a pilot continues to press his priority button, the a/p is disconnected and his side stick and only his is in control. When no button is pushed but both sidesticks used at the same time, the inputs are summed together. In such case the pilots will gett a dual input warning.
If a pilot then presses on HOLDS the button, he/she has has full priority?
Correct!
If so, can the other side-stick take control whilst the opposing button is pressed?
Yes, he can, by pushing his priority button again. It has been said before:
Last pilot who depressed and holds take-over button has priority
If a pilot presses and HOLDS the button for more than 40 SECONDS, does this lock the other side-stick?
Yes. The other sidestick is deactivated.
If so, how long for and when does it return to normal mode?
Indefintely, unless the priority button on this deactivated sidestick is pressed. Then it's active again.
... and will the system ever IGNORE both inputs in a certain situation?
No, dual sidestick faults are lethal
If a pilot presses and HOLDS the button for more than 40 SECONDS, does this lock the other side-stick? If so, how long for and when does it return to normal mode?
Why would you ever disconnect the other sidestick? When there's a sidestick fault (false internal contact with unwanted input or when a pilot drops dead on his sidestick. 40 seconds seems reasonable. It can be returned to "normal" mode by pressing the priority button on that sidestick. Something you wouldn't do if there was a fault, but might want to do if the fainted pilot is removed from the controls. ;-)