Forced password resets and legacy accounts

Could I please get some advice on forced password resets and legacy accounts (in this case a 'blueyonder' one)? I am aware (from reading around here and from earlier advice 5 years ago) that Virgin Media have brought in this "third party email" requirement for the username.

My question is why was my ability to login perfectly fine up until Monday, when it changed without warning to the following screen when I attempted to login:

Update your password. We've updated our password rules to make signing in even safer. To sign in, please change your password to one that follows these new rules. "UPDATE PASSWORD"

When I attempted to do this, I got:

Contact us. You can't reset your password online right now, as you don't have additional details set up on your account. To reset your password, please contact us and one of our team will help.

I eventually got through to someone on the Live Chat, and they explained that there was the additional problem of needing to change to "third party email" requirement for the username.

My question is whether the two are connected at all? Why, when people were reporting that this "third party email" requirement for the username was being required of them many months ago, have I not been asked to make this change before (I would have done this immediately if I had been notified)?

My suspicion is that the "forced password reset" is a way to force legacy accounts to make this change. But it has proved hugely disruptive for this to happen without any warning.

From my Live Chats so far, I am told that all my emails will still be there (yes, I am a paying VM Broadband customer and the email is associated with the account, there has been no problem with that for many years). This is really important to me to be sure about: will the emails all still be there (some are backed up, some not, yes I know...). I am going to be putting a 'redirection of emails as backup' in place immediately once I am back in.

Could those who know about this please advise exactly how this "third party email" requirement for the username works, and what I should expect when this has been set up? Is it like a "pseudo username" or is it an actual redirection kind of thing? What will have changed, and what will this lead to in the future? Is this a stage ahead of finally withdrawing the email service completely, and if so are Virgin Media likely to give any warning?

Final question (sorry): which of many "third party" email accounts I might have should I chose? Does it matter? Should it be the one that I eventually want to import the emails to, or might there be reasons to avoid that? Can this email I chose be changed later if needed?