Did you try to repair the Sent or Sent Items folder?
Right click on Sent and/or Sent Items folder, then go to Properties and then click on Repair folder
Sometimes mail gets "lost" to Thunderbird on moving, and becomes visible again when you repair folder.
Opening messages with the -mail command line parameter is still possible with a current Thunderbird version (tested on Windows 10 with version 78.6.0, 32-bit).
However, this requires a small change of the module MailNewsCommandLineHandler.jsm, because otherwise a getter-only error occurs when calling the -mail command. The error can be traced via the ...