The background to HMRC Brief 19/2020 states: "This result is contrary to the operation of the VAT system." It is not open to HMRC to say that; only a Court of Law can determine whether a result is contrary to the operation of VAT. In fact, the Brief was released precisely because a Court of Law held that the result was not contrary, etc., etc.

Is there a risk that the UK provider of, for example, accountancy services, will be required to register for VAT in the Member State where the private client is based?

Important point. I have spoken with some businesses whose paperwork is in physical files, so they cannot complete their Returns. As you say, lockdown will actually prevent some businesses from submitting Returns.
I would suggest that HMRC should allow a reasonable excuse defence so that surcharges do not arise. The key will be to collate a simple factual narrative of what happened, with dates.

I have heard your fourth example being provided as standard advice from the HMRC helpline for many years. This can have serious implications later on, with the taxpayer unable to reverse his decision to opt to tax.

It’s a pretty obscure piece of legislation that won it. And I should know, as I represented the claimant!

Excellent article.
I make frequent use of the Manuals. I don't always agree with them, of course! But they are an essential resource for anyone trying to find their way through the vat maze.

MTD solve carelessness?
No!
I uploaded a car park ticket. The software automatically posted it to Xero. But it used the time of arrival as the value, so massively over-claimed VAT. I happened to spot it, but would not always have done so.
Had I used my previous manual procedure this mistake would not have occurred.

Fair comment, Alf.
I did make a deliberate decision some years ago not to try to provide comprehensive guidance in my blog, but to raise issues for practitioners to pursue themselves.
Where Accountants have specific issues with individual clients, they are always welcome to contact me for further advice; and some do so.

As is so often the case, had the taxpayer sought suitable advice at the start of the project, he would have secured his VAT repayment, and avoided costly and long-drawn-out correspondence with HMRC, let alone the Tribunal.
(Did you spot who was the Tribunal Member?)

This is a real concern for many advisors.
One additional factor is that, having determined that the taxpayer made a 'deliberate' error, HMRC can publish his/her details in their quarterly report, subject to the £25,000 threshold. This puts the taxpayer's behaviour right into the public arena.