With regards to wheelchair accessibility and reasonableness I do think that TPE have a strong argument that their actions are reasonable (if not ideal).
Firstly, and perhaps most crucial, this is a short term change. The proposal is that these will be introduced in at the end of July (or even as late as October) and used for a handful of months prior to the arrival of the Mk5s which will be fully accessible. The introduction frees up additional rolling stock within TPE, avoids taking drivers/guards away from service trains for the entirety of their LHCS training and will help to relieve overcrowding. The cost of modifying the vehicles for that short period of time to meet the full requirements of the PRM TSI is unlikely to be seen as a reasonable expenditure.
Secondly, the mitigation. The staff briefing linked
earlier (thanks to
@kingqueen ) explains that for passengers between Liverpool and York they will be allowed to take the next available service. Between York and Liverpool there are two trains per hour (for a passenger wanting, say, Huddersfield there are four trains per hour) meaning that, worst case, a wheelchair passenger will have to wait thirty minutes for the next service (helpfully the trains are actually thirty minutes apart). Between York and Scarborough where the wait could be at least one hour then a taxi will be arranged. Again, this is far from ideal, but it does not seem an unreasonable mitigation considering the short term nature of the change being made. I would assume and strongly support that anyone who was delayed by at least thirty minutes as a result of this would also be entitled to Delay Repay.
If TPE were saying that from now on wheelchair passengers will have to put up with the above service then I would quite agree that seems wholly unreasonable and I would hope that TPE could be persuaded either on their own or via legal means to change their plans. However, that is not what's happening.
Looking at what is actually happening then it seems to me that weighing the options this is not an unreasonable change and neither is TPE's proposed adjustment unreasonable.