PIP- Appeal for tribunal denied, told to make a new claim due to Covid-19

In October 2018, I applied for PIP stating depression and anxiety/panic disorder as my health conditions. My application was rejected despite medical evidence from a GP. I asked for a mandatory consideration in May 2019, along with another doctor's note and evidence from my university including absences, my disability support plan from claiming Disabled Students Allowance, copies of emails I've had with tutors discussing my mental health and how it affects my studies etc. Again, I was rejected.

In August 2019, I appealed the decision and received a tribunal hearing date for January 2020. I was unable to attend because it was in my home town and I was away at university and could not travel back. The hearing was cancelled and I was told I'd be sent a letter when a date was set for my tribunal near my university. I sent them my availability dates/term dates and my university address via email, as asked.

I didn't hear back from PIP so amidst the Covid-19 pandemic, I emailed them in April 2019 asking for some kind of response as to when I'd have a tribunal and how that would be set up, seeing as my mental health has only worsened since Covid-19 (I'm asthmatic and will continue to isolate until there's a vaccine for the sake of my health). I received a reply saying my request to change venue had been forwarded to the wrong city, and my appeal was made "non-listable" because it had gone to the wrong place which was why I had been waiting so long. They then made my application "ready to list"

Today, 2 months later, I have finally heard back from a DWP case manager. They used the email I sent them against me, stating that my further evidence (I assume they were referring to the email?) indicated a worsening condition caused by the lockdown. Therefore, they want me to make a new application with MORE evidence with dates to show that my mental health deteriorated (as if that is in any way able to be documented besides a feelings diary which they'll disregard if I try to put forward as evidence). No one has seen a doctor in months because of Covid, what evidence do they expect me to provide? 
They have also used my recent (May 2020) universal credit claim to entirely discredit my student status, despite being a third year undergraduate and being advised to apply once my university officially closes, which was in May, by the student money advice team at my university. I still have yet to graduate because of Covid-19, however. To my knowledge, student status or being on other universal credit  does not have any effect on entitlement or the amount you're paid because it's not means-tested. So I'm very unsure why they felt the need to bring it up in response to my appeal for a tribunal.

What do I do now? I feel like I've said absolutely everything there is to say, I have three doctor's notes from two surgeries, I've been on the maximum dose of two different antidepressents for 5 years which they have evidence of, I have a letter from my parents stating I'm mentally unwell, they have my Disabled Student Allowance report as I've been on DSA for the last 4 years, what is left to give them? 

If I give up and make a new application in the hope that they'll accept it with Covid-19 as further evidence(?), I'll have no chance of the backdated payment from October 2018. It will mean that 21 months of having to relive the worst parts of my mental health over and over again means nothing. I don't know what to do, who to talk to or email.

Any advice on what I should do next would be greatly appreciated, thank you