Thank you both for you for your patient, helpful guidance.
Before I went to bed last night I thought I'd created and posted a brief reply. Clearly I was not attentive enough and it was 'lost to the ether.' Unfortunately I did not compose that one in Word and paste it to the forum like this one.
1) I want to report that I 'solved' the problem of POP mail not downloading from the Sonic server. I checked all the settings and the Connection Security was set to None. When I changed it to SSL/TLS I was able to download the emails that accumulated at Sonic.
2) How do I get the mail client interface to display the inbox in the Onemain profile. I would like to apply myself to sorting it and cleaning it out. Once I've done that I will apply the same for each of the other inboxes in the existing profiles. Then I will work on combining all of them into one master inbox which I will endeavor to maintain more diligently.
3) I'm not clear about the 'standard' structure of the Folder Pane. I believe we are using the term Local Folders differently. Somewhere in the distant past I 'learned' that this was the place to create subfolders and sort mail from the Inbox to keep it emptier. I've uploaded some images to Imgur, a 'current' one and one from 2015. You will see that there is a Local Folders entry at the same 'precedence' level as the Primary Account at the top. The sub-folders shown are only the top of the tree.
https://imgur.com/a/tGZ9HRY
4) I think I understand the elegance of Dave's suggestion of having all the sorting folders under the Account Level. At some point in the past, when there were messages stating that the Inbox was full, I created an Inbox folder under Local Folders so I could simply sort by date and select several months worth of messages and drag them to the Local Folders>Inbox folder for later sorting. I see that this is not a good strategy for a procrastinator/hoarder.
5) Has anyone ever created a 'tutorial' on 'best practices' for arranging the Folder Pane. As I looked at mine and read Dave's suggestion I could see merit in something like a simple alphabetical breakdown that when expanded reveals all folders A-E, F-J, etc With many subfolders, I've found it tedious to file selected files to drag from a Name or Sender sorted Inbox into the appropriate subfolder in Local Folders since they do not all fit on the screen.
6) As I delete mail and even subfolders should I see a corresponding reduction in the size of my Profile folder? Treesize Free tells me that ~40% of my available System Disk (a 128 gB SSD) is dedicated to Windows and 19.3% to Mozilla Mail.
Thanks again,
baumgrenze