i think you'll find VOSA may well have closed their depot, gone the same way as the DVLA branch offices Regarding the courtesy car, i have never expected a loaner to last for the duration of a repair - it is a courtesy to provide a quick-fix and allow you to source something to keep you going if the dealer's action does not result in a speedy return.
i'm not aware of any manufacturer that will provide a non-time limited loaner until the fault is identified and repaired. Yes, I do have 2 cars and I pay for the privilege (no other drivers) but I need redundancy. i would certainly expect my battery lease to be rebated for every month I was unable to use my Fluence but a loaner? Nice, but not an obligation.
You see this type of complaint with contract mobile phones - there's something not right, and they want a replacement to keep them going. is this any more 'reasonable'? Nope, a loaner is not a right but a 'courtesy' and isn't open ended.
if I knew I would get a replacement vehicle on request, I'd ditch my backup and save a fortune, but they won't and, as you have found out, is causing you even more high blood pressure than if they didn't
i certainly sympathise with your predicament and wouldn't wish it on any forum member, but the loaner is a side issue and an irrelevance in the full state of things - either the dealer's EV bod has reached the end of his understanding due to the lack of fault codes, or Renault ZE is not providing the requisite next level support.
To my mind, this is far more worrying.