Working with fresh v14.2 and r8647 installs let me identify at least part of the issue: including a chat tab as part of the Main Interface window. It doesn't matter how I do it, either a) adding the "ChatManager" setting to the "initialDesktop=" property in the GLOBAL_prefs.txt file, or b) dragging and dropping "Loathing Chat" to the "Startup in Tabs" list in the preferences GUI, will generate the exact same debug log. The only difference between this attached debug log and the one generated by the v14.2 release is the KoLmafia version number in the header.
Then I copied the 14.2.exe and 8647.jar back into the old folder that I had been using with v14.1, and removed "ChatManager" from the "initialDesktop=" property line in the old GLOBAL_prefs.txt file. Now both v14.2 and r8647 are able to load the Main Interface window without logging in to the KoL server, and most importantly, without generating a debug log.
Sorry I couldn't include this in the OP, but better late than never, right?