Migration of email between iCloud and Thunderbird?

Question

History: I have an email address on a domain that has limited SPAM and AV filtering. Thunderbird is installed on a Macintosh as my email client on which has over 275 folders and filters for the Inbox. To provide better anti-SPAM and AV services I setup a GMail account that retrieved the email from the ISP, cleaned it and left it for retrieval in the Inbox. Due to space constraints due to email, which are used for reference purposes, and the fact that I had so many local folders, I decided to have a Ubuntu Dovecot mail store setup on a virtual machine under Parallels. The mailstore used fetchmail and retrieved any email from my GMail email address and stored it locally. I only had my email on one machine as didn't bother enabling the service to an iPad/iPhone.

All worked perfectly until....Catalina needed to be installed.

I downloaded all the email(185k across 275+ folders) from the local server into my Thunderbird profile and then did the upgrade.

Dearest Catalina, TOTALLY wrecked my VM under Parallels but thankfully I still had all the sorted email and folders on my TBird profile.

I have implemented a rule with my domain email addess to automatically fowarded ALL email to my iCloud address.

The ISSUE: Moving toward 2020 I needed to be able to access all the email stored on my TBird 'iCloud' profile from an iPad/iPhone.

Simple enough: Increase iCloud storage, import folder structure and email to iMac Mail, setup new account on TBird for iCloud Mail and viola, all email would now not only be on a single machine but also on the iCloud and could be accessed by iPad/iPhone.

I purchased additional storage on iCloud, fired up iMac Mail and started an 'Import Mailboxes'. The import completed after 3 days but not all the destination folders were populated despite the source having content.

Since Oct 2019 no email has been sorted out of the email Inbox linked to both iMac Mail and iMac Tbird as the folder structure for storing the email is not stored on the iCloud.

I would re-create the folders manually but given the complexity of the folder structure, would rather not and forwarding all the email is not acceptable to get into another folder structure.

Questions:

1. To transfer the folder structure stored in my Thunderbird profile to my iCloud profile?

2. As both TBird and Mail use mbox apparently can I move the mbox and .msf files from my domain email profile folder and insert them into the iCloud email folder?