My mod stack was working perfectly ok. I use SKSE to load the game through a short cut on the desktop. Before the tech issues, I had Solitude Docks District & Better Docks mods working fine. Then I made the mistake of trying to test another mod called Solitude Furniture store which was in the same area.

BOSS doesn't have any issues with the mod load order. But when I tried to launch the existing save game with this new mod enabled (which only had an esp, bsa and no loose leaf files), the working save game immediately CTD.

So I disabled and deleted the mod esp and bsa from the Data folder. But when I went to reload the existing save game, the game now immediately CTD??

The only difference from before was that I had been initially launching the game with Steam in online mode.

When the save game failed to launch because of the conflict with new mod, I removed the mod. Then changed the Steam menu setting so Steam would launch in offline mode (without the mod)

A save game was never created with this mod, so then why is the game having issues trying to load the former save that was working? I verified that all mods that were originally activated in the mod deck hadn't been disabled by BOSS etc. But the game still CTD?

Are there crash logs somewhere or something I could tune in the .ini files to see what's CTD the game?

If I understand your issue correctly, it sounds like you have removed a mod from your game (i.e. you deleted it) and then the game immediately crashes to the desktop. You will need to reload a save file prior to installing the mod that caused the problem, because leftover scripts are likely permanently coded into your current save file. This commonly happens when removing a mod from your game, because the save file becomes dependent on the mod being present, even though in reality it is no longer present.

Edit: from rereading your post, it appears that you've already tried what I said above. You may have to remove the ini files from your skyrim folder (Documents > My Games > Skyrim) and let the game generate new ones.