I’m in a similar situation.
I have issues using the phone and I find the people on their live chat are basically clueless.
After a failed attempt on live chat I went back on and purely asked for an email address.
I then formally complained by email about my issue. They tried to call me in response so I just ignored it and they emailed their response.
I’m beginning to find the whole MB system is flawed and not supportive of disabled people.
They get bad press especially whilst under a conservative government as they promote the idea of a corrupt company giving out free cars and taking government money.
In that light it may be contact with an MP or threaten the use of the press to them to try and get them to buck their ideas up.
If they are not fit for purpose perhaps it’s time to rock the boat.