I have bucketloads of empathy with both your situation and the madness of how banking is operated, especially HSBC and Santander. They’re totally ignorant of the damage they cause by poor communication and this “we’re always right” attitude.
I also agree with your various tips and advice — I would add additionally that any company having any kind of payment issue should absolutely talk to suppliers. We had a customer once where they were going bust, but the director was very open about the situation and how they ended up there. That honesty meant we let them keep the services running, and when he setup fresh, we agreed to supply the new business, and he in turn said he would pay the remaining costs from the old agreement in time, and true to his word, he did. You clearly did the same where you could of course, but it is a good general tip for sure.
For all those having similar issues, you need to go to a branch and start making pretty vocal fusses and take direct action — it’s your business, your money and the bank needs to be held to account. Get it in the press, whatever it takes, as sadly nothing else ever seems to work with these companies anymore. It’s just computer says no and case closed.