Thunderbird Account duplication with settings - How can I copy an account, then modify that new copied account, while totally offline

Question

Trying to SET UP four IMAP Gmail email address accounts, so I can just download the headers without messages, so I can then manage mailboxes, delete unneeded emails, etc.
It seems very difficult, since TB wants you to log into your account, BEFORE or DURING the process of creating ANY account. I'm not trying to copy or move any mailboxes, headings or messages as some others have asked for advice. I'm also not trying to duplicate a complete copy of all the user settings, i.e., Thunderbird's global profile settings, for a new PC.

I JUST want to copy any ACCOUNT and the SETTINGS, without mail, while offline.
So, how can I just copy an existing account, while keep all my settings rather than recreating them by hand? I do not want to connect until I've had a chance to verify the settings won't cause problems. Obviously, I will have to change the email credentials, email address, etc. But, I must do so while offline.

My experience is from Eudora, where duplicating an account with settings offline was with a single click. Once done, you could simply rename it, change the settings, etc., and THEN connect. With TB, it appears it was never designed to create any account offline, let alone duplicate, and then modify that newly copied account.

How can I copy this account manually, safely, without too many problems? Since I'm assuming there is no featured way to do this, the next question is how can I duplicate the account offline, without connecting until I've modified everything perfectly. I can copy whatever files and folders, and probably change some of the key parameters. If I have to first create some dummy account, then copy the files, modify them, and then connect, then that's what I will do.

Why is this so important?
I'm just trying to set up a GSuite / Google Apps "account" that is identical to test account. TB says that they "will configure Gmail properly", if recognized. This is not the case, since I found specific Google recommended settings that are not the same as when TB pre-sets them. (See: Gmail recommended IMAP Thunderbird settings) If the settings are not quite the same as the account I worked hard to test, I could be making fatal, permanent changes to Gmail's IMAP server in the new account. Worse, I could start downloading and go over quota, before I knew I'd made a mistake.

Where are the individual account settings kept?
Hopefully they are not all in the profile file with all the rest of the settings, and they are kept in their own file in their own folders. Last question -- What issues am I likely to see when I try to manage +250,000 emails in a single mailbox? Will it choke for performance or other issues, corrupt the IMAP databases? ie, is Thunderbird really stable enough for today's email?

Answer 1

I've been busy trying to get the account settings corraled into one place, or researching other methods of accomplishing what I'm asking. Here are a few of my findings so far:

TB is NOT Eudora... and it isn't architected nearly as well or cleanly. ALL the account settings for all accounts are held inside the Prefs.JS file. Worse, all the settings are not isolated in a single section. When you delete an account, it does NOT clean up the JS file, nor does it clear out all the files and folders created when you do.

I'm starting again from scratch from uninstall. Unfortunately, you can't create an account offline, at least for IMAP. There is a "Manual Setup", but unfortunately, you still have to connect the account to finish--and this means you can't change the settings, until after you've started downloading mail.

I think part of the reason for this was that the IMAP folder needs to direct TB how to configure their settings, files, folders, and mailboxes, which of course, it can't do until you create an account.

My plan is to do a fresh install, change a few settings, create my account and connect, and immediately go offline, make all my settings changes (by hand, for each account). And then go online to get my headers-only. I will try to report back, what I've learned. I'm also trying to keep all my local files on an alternate drive. So that too should be an experience to set up, before, during or after I install and set up any accounts.

I'd caution against trying to even write a tool to copy an account and or individual account settings. There appear to be a number of dependencies on other files, other folders, and some of these are in common to other accounts. RIP Eudora...