UC Migration and a PIP Backdated payment

I'm helping a relative with their migration from ESA (Support Group) to Universal Credit following a migration letter they received instructing them to make the move. Their identity was successfully verified online during the initial sign-up process.

My relative also receives PIP, and in 2022 they were awarded a significant backdated payment following a change in the law which meant they qualified for a component they previously told didn't apply to them. This covered several years and amounted to a few thousand pounds.

During the UC application process, they were asked to declare money in the account & if any funds were from backdated payments. We fully disclosed the current amount and the the backdated payment which my relative has held on to, believing it was the right thing to do.

A new task has now appeared on their UC to-do list, instructing them to attend their local Jobcentre and bring bank statements showing the backdated payment, along with statements for the last three months. We have all of this evidence, including the original DWP letter confirming the backdated award.

The concern is that my relative is completely unable to attend the Jobcentre due to the nature of their illness.
We have updated their UC journal explaining this, the nature of the payment with dates and asked if it’s possible to submit the requested evidence remotely, either through the journal, email, or an alternative method.
However, the JC appointment is in just 48 hours, and we are extremely worried that a decision-maker may not read the journal update in time, and their claim could be closed for non-attendance.

Can anyone help with advice on;

What happens if the journal message isn’t read before the appointment?
Is there a formal process for uploading documents to Universal Credit or the journal?
How can we ensure my relative is not penalised when they are unable to attend due to a genuine health condition?

Is there anything we can do if the journal update isn't read? I ticked the Appointment button when posting it.

The situation is causing my relative severe anxiety, and as their supporter, it’s been incredibly distressing to see how much it's affected them. Any advice or guidance would be very much appreciated.