Alright! Thanks for posting pictures, that helps immensely.
The pic that shows the starting directories, that's fine if that's showing blank for one or the other, the program simply uses the game directory if you don't have a custom starting directory set, and you could probably have that cleared or not set for regular TQ.
As for steam, the ACTUAL desktop shortcuts appear to be web/file urls (odd setup...) to launch the game. You have the command line argument set, but not the executable and the command line I don't believe is right.
If you placed what you have in the command line argument into the custom executable ("steam://rungameid/4550") it would probably work, but unfortunately as-is the Defiler checks to see if that string exists before it would use it...and something that looks like that isn't truely going to *exist*, and wouldn't be used. If it comes to that I'll make a custom version of the Defiler for you to try with a weburl like that, and if it works I'll include a simple one-click button to set it to use something like that for future use. For now lets try the normal custom executable route...
First clear both TQ and IT settings by clicking both the 'Clear Paths' button for each on the 'Executables' tab. Next open that settings.dat file with notepad.
Set the following settings:
"[itexe]=C:\Program Files (x86)\Steam\steam.exe" (or wherever the steam.exe file is located)
Save the settings file, then open the Defiler and use the 'Run Immortal Throne' and let me know what happens!