Launching the 64-bit-exe results in a black screen/crash at startup

I assume this would be the equivalent of adding the /x64 parameter in Heroic:

This won’t work on Linux with ia32_emulation=0 Kernel argument. To my knowledge there’s no way of temporary disabling the 32bit emulation on Windows, you could however rename/move the regular Grim Dawn.exe and see if your trick still works.

Not really. I mean. It will end up doing the same thing, but you’re launching the 32 bit version of the game and telling it to launch the 64 bit version with the /x64 toggle.
I don’t know how heroic works. If you can’t set the working directory, maybe you can make a script in the grim dawn main directory that launches the 64 bit version of the game and select that as ‘EXE’?

Might also works, if you copy all the content of the x64 directory in the main directory and overwrite everything, but that’ll be very inconvenient with updates and the like.

Will not lie, I won’t be testing that - but even so, like you pointed out, that won’t do anything when an update rolls around.