OP, you're leading us astray:
The PCN does not refer to 'grass verge';
Their rejection does, but only because you raised the issue, it then states and re-states that the issue and contravention relate to the DYL.
Pl get away from grass verges, you cannot park where you did because of the DYL.
I cannot see any substantive defence here and the discount expires on 10 August.
Yes, it's possible that the traffic order might be flawed and yes, they might reissue the discount after rejecting formal reps, which themselves might yield substantive procedural grounds.
Perhaps. But what you do know is that the discount expires on 10 Aug.
If you're the registered keeper with current DVLA details then you have to decide whether to carry on.