1 - Is this your question on Microsoft Q&A or just a very similar situation?

Outlook 2021; 4 Pop Email accounts share 1 Data/pst file. - Microsoft Q&A

2 - My guess would be that your 2010 was using POP3 for all the accounts and was configured to use the same PST. But when you setup the accounts in 2021 they were added as IMAP, which can not share the same OST. Before we continue, please confirm by doing File -> Account Information/Settings -> Account Settings and look at the "EMail" tab, "Type" column. What do you see?

3 - 8GB worth of data

To get access to all the mail in the 2010 PST you just need to "Open" it. This will add it to your current profile as a top level folder and you should be able to access everything in it.
Open and close Outlook Data Files (.pst) - Microsoft Support

4 - profile, (named "test", which I would also like to change)

https://www.extendoffice.com/documen...file-name.html