As to question #3, no PST File showing - just OST.
Am I understanding things correctly that you encountered the error ""Mail Deliver Location - Outlook (97-2002) Data Files (pst) cannot be used when more than one Exchange account is configured" when you tried to change the <default data file> from one OST file to another?
If yes, am I also correct in assuming that the OST file was (is?) set as the <default data file> belongs to an IMAP account and the OST file you wanted to change the <default data file) belongs to an Exchange account?
If the answer to the above is: Yes then it is quite likely that either there is corruption somewhere. FWIW, I have some 25 different email accounts configured in my primary Outlook profile using Outlook '365 with a mix of POP, IMAP and Exchange accounts and don't encounter any errors changing the <default data file>
One final question. How can I fix that broken ScanPst file that won't scan because of a missing dll file? Will a Repair on Office do the trick or is that totally out of the question.
Getting a DLL error for ScanPST certainly indicates that something is wrong. Doing an <Online Repair> of Office would certainly resolve that but certainly is curious as to why you got that error in the first place. Makes one wonder if there are other issues at play so repairing Office certainly wouldn't hurt things.
Now for some comments and a suggestion
#1 - When configuring email accounts in a new Outlook profile, the data file for the first account would be set as the <default data file>.
#2 - You really don't want to have the OST file for an IMAP account set as the <default data file>. An OST file from an IMAP account set as the <default data file> would contain "This Compute Only" folders (contact & calendar). The problem with storing anything in any "This Computer Only" folders is that if Outlook cannot find / open the OST data file, it will automatically create a new one, sync the email folders with the server but the "This Computer Only" folders will be empty (ergo the data in those folders would be lost"
#3 - Instead of worrying about prior posts in this thread and at the risk of being repetitious, my suggestions would be as follows
a) repair Office
b) if you haven't imported the contacts from the PST file into a contact folder belonging to an Exchange account, run ScanPST on the PST file you want to import
c) create a new Outlook profile using the Control Panel Mail app
d) configure the Exchange account where you want to store your contact / calendar data as the first account so that its data file is set as the <default data file>
e) add the rest of your accounts in any desired order - doesn't matter. If you want one of these account to be the <default email account> - you can set the account you want by
<File> --> <Account Settings> --> <Email> tab --> select the email account --> click on <Set as default>, This is complete separate and distinct from what data file is set as the <default data file>.
The <default email account> governs what Outlook uses as the default <From> address when composing a new email message.
f) import your contacts from the PST file originally referred to into the contact folder of the first Exchange account (step <d>)
That should end up with a clean Outlook profile with no need for any further changes