I have installed and configured all of the prerequisites and this tool on my Macbook to experiment with homerow mod setups.
This is working great thanks to the keymapper.conf example provided in the Discussions here.

But, I have one problem I cannot solve. It seems keymapper, or some interaction with the Karabiner VHID Daemon process, is swapping Backspace for Delete. If I hold fn(🌐) on the Macbook keyboard then hit the Backspace key I then get a Backspace. Otherwise I get Delete.

This is the opposite of standard behavior. I know the key is literally labeled "delete" on the Macbook's keyboard. But this key normally behaves as Backspace, unless fn is held down when pressed. Then it does act like a Delete.

I tested with the https://www.toptal.com/developers/keycode tool with keymapper running, and indeed it does register as Delete when I press what is normally Backspace and the reverse when I hold fn+Backspace.

To rule out any issue with my config, I tested with a completely empty keymapper.conf file and this issue still remains.

And when I kill the keymapper process, the expected standard Backspace behavior is restored.