Hello.
I enabled checkboxes for all folder types in Preferences > Folders > Folder Formats then double clicking each one and making sure the checkbox next to Display was checked, then checked the checkbox next to Checkbox mode. (I don't know if there was a better way to do this...)
Then, I click once on a file to highlight it. I'm 99% sure no files were checkboxed. When I pressed Del on my keyboard, it asked me if I was sure I wanted to delete 2 files. Very odd...
So then I asked ChatGPT for help (BIG MISTAKE) and, long story short, I deleted the functions for Del/Shift+Del for both Toolbar:Operations and Toolbar: Image Viewer in Customize > Keys.
I cannot find what the default function definitions should be for these. When I click the hamburger menu in the top left of the Customize window, Factory Reset is grayed out. And no matter how many times I hit Ctrl+Shift+R (undo all changes) the default function text does not come back. 
I also cannot find what those default functions should be using the DOpus help or this forum.
So:
Firstly, I need to know if I enabled Checkbox Mode properly.
Secondly, I need the function code for Del/Shift+Del for Toolbar: Image Viewer
Thirdly, I guess I need the function code for Del/Shift+Del for Toolbar: Operations.
(I will just uncheck this, as I want to create my own function I think, see below)
Fourthly, I want to create a new keybind (I think?) that has the function of deleting/permanently deleting only the selected (highlighted) file(s), ignoring all checkboxed files.
Phew. Thank you very much for any help you can provide.
(P.S. I will absolutely be purchasing DOpus when I can afford it - I expect to be employed again within the next couple weeks, and part of my first paycheck will be going to you, I promise. DOpus is my #1 favorite piece of Windows software, and while I could use it for free, I want to support you in continued development!)