@tyersal I'd suggest going back there on foot (park in the vicinity or go via public transport) and get some proper photos of the signs. Please remember it's only the sign for the bay that you stopped in that counts, signs in other bays are irrelevant no matter how clear they are.
The PCN should have come off-hold today, and it went on-hold only 2 days into the 21 day discount period, so the discount should not expire before 2nd June assuming the TFL computer actually applies a hold and does not skim any days off the discount period.
On the other hand the statutory deadline to make representations is 22 May, so you cannot go past that, and it might take till 23 May to get the traffic order back.
In the circumstances, I'd suggest you make a representation now in order to protect your position, it doesn't really matter what you say as you just need to buy time, a bare assertion saying that no contravention occurred will suffice for these purposes.
The rejection letter will reoffer the discount for a further 14 days and as it will take a few weeks to come through, by then we should have the traffic order. If you're lucky there might even be a procedural impropriety in the notice of rejection.