Sorry I'm so late in responding.
I had previously misread your post and didn't realize you had gotten it working with GIMP. Cool! Now onto other things:
First and foremost, you probably don't need to deal with even running the installer. The installer is a 7zip self-extracting archive. That means you can use the 7z command to extract it, and 7z is much more likely to be available than file-roller, and would potentially require less dependencies to be installed. Here's what I'd do to check if 7z is installed.
[ -x "/usr/bin/7z" ] || POL_Debug_Fatal "$(eval_gettext 'The command 7z, which is required for installation, is unavailable. Please install the p7zip-full package.')"
Next, here's what you need to extract it, assuming your current directory is $POL_System_TmpDir and you've made the directory "nik" inside:
7z x -onik -y "$INSTALLER"
This takes a while, so you probably want to add a POL_SetupWindow_WaitBefore before it. Next, you can copy files. I'd warn against copying the entire directory containing the Nik Collection programs, since that includes 64-bit binaries and this is 32-bit Wine. So you'll have to make some directories corrosponding the the ones that the programs might expect.
As you said previously, the name of the folder containing the application files themseves contains invalid characters, so I can't paste them into the message, so you'll have to figure that out on your own. Basically, though, the directory structure that you'll want in the $WINEPREFIX/drive_c is:
$PROGRAMFILES
|-- Google
|-- Nik Collection
|-- Analog Efex Pro 2
|-- Color Efex Pro 4
|-- Dfine 2
|-- HDR Efex Pro 2
|-- Sharpener Pro 3
|-- Silver Efex Pro 2
|-- Viveza 2
Note the use of $PROGRAMFILES: this is because Wine translates the name of the Program Files folder based on locale.
I'm not sure exactly how your script worked, frankly, because it didn't seem to copy anything to the Program Files directory. (Maybe POL_Shortcut searches throughout the whole drive_c---I can't remember.) But that doesn't really matter anyway.
One more interesting thing: rather than telling the user to drag a file onto the shortcut, here's a nifty trick: use POL_SetupWindow_browse whenever it's not being called from GIMP! Here's what you'd need. (This example is for Dfine 2, the backslashes are escape characters.)
POL_Shortcut_InsertBeforeWine "Dfine 2" "[ -z \"\$@\" ] && POL_SetupWindow_Init"
POL_Shortcut_InsertBeforeWine "Dfine 2" "[ -z \"\$@\" ] && POL_SetupWindow_browse \"$(eval_gettext 'Select a file to open in Dfine 2.')\" \"Dfine 2\""
POL_Shortcut_InsertBeforeWine "Dfine 2" "[ -z \"\$@\" ] && POL_SetupWindow_Close"
POL_Shortcut_InsertBeforeWine "Dfine 2" "[ -z \"\$@\" ] && set -- \"\$APP_ANSWER\""
