Yes, you can use dual channel if you want, but a kit of four is an add configuration as well. I would go with a 2x4GB kit, unless you absolutely need more than 8GB.
The issue you experienced with erasing the Marvell Firmware causes the board to stall looking for that controller, as it's enabled by default, so that may be why it doesn't work properly for you until you fix it. It just flashes or erases the full or sectioned parts of the Marvell firmware, nothing to do with the Gigabyte BIOS or IDE/AHCI controllers in the Gigabyte BIOS, but the lack of firmware in the Marvell controller likely does cause issue since the BIOS would be looking to load that ROM and Firmware at the same time or before the normal other hard drive controller.
I think your issue will be fixed with other memory, unless something else is causing your slow memory scan. I tested with two different kits that day I tested, no memory scan issues or anything else as you can see in the video. Go with 8GB or 6GB/12GB, you can always sell the 6GB/12GB kit if you want to go dual or quad in the future, or use 4GB/8GB of that and keep one as spare
