If you are using Crowbar to publish to Garry's Mod workshop, delete the addon.json file (the one mentioned in the error message). Try again to publish via Crowbar. If the error shows up again, post (copy-paste) the contents of the addon.json file here, and I will find out what is wrong with it.

The error messages explain exactly what is wrong and what the causes are. There is nothing else to explain.

Likely the QC file has an opening brace "{" for $model, but no closing brace "}" at the appropriate spot.

This is likely not a bug with Crowbar, but with Garry's Mod or Steam.

Without seeing Crowbar's upload log, I can only guess that it is hanging or freezing when creating the GMA file. Crowbar calls gmad.exe, the packing tool that comes with Garry's Mod that turns a selected folder into a GMA file.

If the GMA packing is the problem, you might get a better result with the Garry's Mod "x86-64 - Chromium + 64-bit binaries" beta (via Steam Library > Garry's Mod > Properties > Betas > Beta Participation) and setting Crowbar to access the 64-bit version of gmad.exe (via Crowbar's Set Up Games tab).

Try putting the QC and related files into a folder that does not have a dot (.) in the folder path, such as "C:\temp". The model compilers seem to have trouble when folder path has both a space and a dot.

Sounds good. (Pun intended.)

The MDL, VTX, and VVD files together make a single model. You keep those files together in the same folder and select only the MDL file to decompile. Crowbar gets the necessary info from all three files based on the MDL file name. That is why Crowbar labels the input box as "MDL input" and not "VTX input" or "VVD input".

The syntax in the duckgun.smd file is incorrect. Maybe the material name is too long or has invalid characters.

Based on the call stack, it looks like the person opened Crowbar and it tried to do the auto-check for newer version, but could not call a certain function due to missing piece of the DotNet Framework.

Make sure you are using Crowbar 0.74 (the latest).

This sounds like a Steam error and there is nothing you can do but try again later.