OK, so your parking session ended at 15.47. The PCN is timed at 15.58, 11 minutes after your session expired. This means the PCN would have ordinarily been served correctly, because 10 minutes grace is allowed after expiry before a PCN can be served. However at 15.58 you had started a new session, which I suspect had yet to be downloaded to the CEO's hand-held terminal.
However, your case turns on the interval between the first session ending and the next one starting. You state: -
I tried to find another parking spot down the street as I was running errands and attempted to find a more convenient parking spot / location. But unfortunately there were none so I returned back to the same parking location.This is when I arranged for the second parking session while I walked to the needed store.
I take it this means you walked up and down to find another space available without moving the car. So having not found another space, you walked away to the shop, paying for another session while you walked instead of doing this by the car. So it seems to me that the CEO was printing out the PCN at the same time as you made your second payment. Except, of course, there is an interval of 11 minutes of non-payment. If you had extended the session before the first one had expired you would surely have been OK. There is nothing on the sign to say "no return in <n> hour", so the implication is one can extend a session. You however did not.
I would wait and see what the others say, but I don't think this is an easy win.