As I noted, the other similar MET case is running fewer arguments than yours, as the OP in that case is the registered keeper of the vehicle, you are not.
The argument I have added in around hirer liability usually has a good success rate. We can't offer guarantees as POPLA assessors are prone to making odd decisions, but I'd say it's one of the stronger arguments.
If I were a betting man, there's every chance MET will realise they've not got much hope and pull out before they have to pay the POPLA fee.
Either way, if the worst happens and the assessor finds against you, this decision is not binding on you.
Also, should I adjust this [NAME] (Registered Keeper) (Appellant) to (Hirer) instead of Registered Keeper?
Yes you should, well spotted. I'll amend.