Apologies if these are Microsoft rather then Virgin questions.
They are certainly interesting questions.
The first thing to say is that the IMAP protocol only covers the synchronisation of emails and email folders across devices. It does not cater for syncronising calendars and calendar information. So the simple answer to your question 2 is that you need to "be using something else". I know that some people use Google Calendar but there are also several 3rd party products designed specifically to sync calendars across devices. A bit of online research will show you what's available.
As far as question 1) is concerned. The Virgin Media email service is a just a basic service and does not include any calendar function whatsoever.
However, I assume that what you refer to as your "IMAP ntlworld.com calendar" is just the standard calendar in Outlook 365. However, that's not necessarily linked to any specific email account in Outlook.
You can set up more than one calendar in Outlook (see here https://support.microsoft.com/en-us/office/create-additional-calendars ) which may well be (almost) what you are looking for. At the moment I am still using Outlook 365. I had not previously tried setting up a second calendar in Outlook but I have just set up a second one as a test. The default calendar view in Outlook seems to display the two calendars side by side which would work for me (if I wanted two separate calendars that is LOL).
Hope that helps a bit.
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