Their evidence is the PCN and CEO's notes supported by photos. This shows your car parked on DYL, a first observed time of 7.43 and a contravention time of 7.45. From your account you did not see the CEO on your return, therefore it's reasonable to assume that you were out of sight from before 7.43 to at least 7.47, allowing time for the CEO to produce, sign and serve the PCN.
IMO, your reps should be constructed around this background.
Could you flesh out 'I pulled in briefly to return to help him and gather his bags' which implies that you saw him from your car and if you parked to assist him on the spot then as you could see him from the car then surely he, and then you, could see the car from where you/he were. Perhaps you were initially waiting further into PGA and then as you wanted to leave the car to check on your son had to find the nearest vacant kerbside location which you could see some way ahead albeit on DYL?
PCN issued 9th, therefore last day of 14-day discount period(and possibly risk-free period for making reps*) ends on 22nd.
You have time to draft reps which can then be reviewed.
*- pl post the back of the PCN then we can check.