I think we've had this conversation before, it's not made to work with drive letters and I don't think you can do that in DT at all... At least last I checked DT required driver numbers in command-line, so your custom CL isn't gonna work (which is why your test mount fails). You shouldn't ever need anything in there, and I would just remove your custom command lines and rely on the automation as I know that works properly.
Your args are correct:
-mount dt, 0, "C:\Users\Retro Dream\AppData\Local\Temp\Unzipped7-Zip\Alien Trilogy (Europe) (En,Fr,Es,It).cue"
It's supposed to use DTAgent.exe (which it is). It's also not made to work with multiple drives.
Can you get it to mount using a command line outside of GameEx? It's sounding to me like an issue with your setup. I'm also not clear why you need an SCSI drive and a normal one. If you have one emulator that needs SCSI, why not just use that for all of them? I'm not sure if that's the issue or not, but for testing maybe try and remove one of them. I still think it's the drive letters you're trying to pass that's breaking it though.