PIP Mobility 50% discount on Road Tax- How do you claim after initial registration with DVLA?

TL/DR at the bottom.

I was recently awarded PIP Mobility at the standard rate, when I was previously receiving DLA I was awarded mobility component but when I moved over to PIP originally it wasn't carried over, sorry I digress.

As the titles says really, I contacted the DVLA and using recorded delivery sent in all the required documents: V10 form, logbook, statement of entitlement and a postal order to cover the cost of the Road Tax. "Covid" was used as a reason that it might take a little longer than usual, which is fair enough, I understand they've had some terrible working conditions during the pandemic.

A few weeks later, my documents were returned along with the postal order (which I no longer have the receipt for) and a letter saying my request was being dealt with. A few more weeks later my refund arrives, £450 less than I was expecting. I'd been refunded for my current car which I'd purchased at the end of Sept 2020 and not for the car I had previous, my PIP entitlement is from 31st Dec 2019.

Another phone call later, I now had to send my statement of entitlement and a letter explaining the situation and giving my previous cars details, again "Because of Covid" it may take longer. I also explained about the postal order and was told it wasn't enough to cover the 6 months and if I got a second PO to make up the difference and sent them with a V10 form that would cover my Road Tax which expired on the 30th Sept.

I suspect this person might not of known what they were talking about, because my notification of Road Tax direct debit arrived as usual and was for the full amount not the reduced rate, so back on the phone. (I'll point out at this time I had 2 letters with the DVLA the first with my Statement of Entitlement sent to the refunds team and the second with £80+ postal order, both sent recorded) I wasn't expecting any news on the refund but wanted too know about the direct debit, I was told the computer was saying my car was taxed until March next year and to cancel the DD at my end just in case, I assumed this meant the PO had gone through and end of....

On Sept 28th a letter arrived from the DVLA containing the PO's and saying the computer says your car is taxed until March next year. So I'm back on the phone with rather less sympathy than previous, trying to find out if it's only showing as being taxed because there's a DD due and of course it is even though I've asked for it to be cancelled more than once. I'm also told that "If you want to pay by DD I have to pay the full amount and then claim it back the 50% reduction from them by sending in my Statement of Entitlement every time I make a payment and that usually it takes up to 8 week but Covid........or I could wait till I'm posted a renewal notice and send in the PO but because it's taking longer it will mean there might be a gap in my tax and because the system can take several days to update, meaning I could be tax-less for weeks if I pay this way". How can this be right?

Are my options really, claim my tax back every 6/12 months which takes over 8 weeks to process or risk being car-less for 2-3 weeks waiting for their system to update?

TL/DR

I'm been messed around by the DVLA and getting contradicting information every time I phone.

How do you go about getting your PIP Mobility Road tax discount after initially letting them know?

PS I am still waiting on the refund for my previous car, meaning including the postal order made out to DVLA and the current tax period reduction I'm owed over £620