This might help clear it up a bit.
To flash the Gotek to HxC, you have to have use the program that comes with your purchase of the HxC software. The bootloader is flashed using a USB to Serial adaptor. Here is a quick and easy description: https://www.ikod.se/hxc-gotek/
Once it's flashed to the HxC bootloader, all firmware is loaded through the USB drive itself. You can then load HxC or Cortex firmware just by putting the .upd file on the USB drive, holding down both buttons and booting it up. It will then flash the firmware into the Gotek. Easy Peasy.
Updates are the same. A new firmware comes out, put the file on the USB and boot it with both buttons.
As to the selector.adf, it is only used for the Cortex firmware. If you use the Cortex firmware, even with the HxC bootloader, you still use the selector.adf file as you did with the full Cortex software.
The HxC has it's own loader. It has a total of three modes: Autoboot, Indexed and Normal. These are controlled by a CFG file you put on the USB drive.
Autoboot - Uses a file Autoboot.hfe that is similar to the selector.adf.
Indexed - You must convert the disk images to hfe files and they must be named DSKA0000.HFE, DSKA0001.HFE, et al. Then you select the correct disk image with the buttons. No additional software is required.
Normal - Requires the addition of another button and an I2C LCD screen. This allows you to browse the USB drive, mount and unmount disk images on the fly. This mod should cost less than $8.00 to add and a little soldering.
Here is an example of one I added:
Hope this helps.