Do you have a spare or backup Ssd or hdd?
I was able to run 2 RX 480 cards by selecting inject Intel, and no inject ATI, no EDID, no Load Video Bios. Once you have the 4100kext mod done correctly and have accleration you don't need anything for the RX cards to get support.
Try this, use EFIMounter and mount your EFI partition. Open clover folder and pull your config.plist out and save in a folder. Now remove it to trash (no EFI folder in clover).
Next go to main partition using the GO tab/Computer/(select disk Sierra is on)/EFI Backups folder. Open and remove config.plist to trash.
You now have no config.plists in either folder. Now download a new clover bootloader and instal. This should give you a MacBook 1.1 definition in system bios in clover EFI folder.
Run clover configurator and mount EFI partition, go to devices and add Intel fake ID 0x19120003, and then go to graphics section and add ig. Platform 0x0a206080. Don't select inject ATI, or Inject Intel. Set any other fixes you normally run on your system but stay away from sound fixes for now. Run the magic wand and select system definition and make a new serial number. Now remove that serial number and replace with your saved config.plist numbers (if the numbers were good and are important to you).
Close clover configurator and then open EFI folder/partition and check that the fixes and IDs are there. Then reboot, at clover screen check again that you have fixes in graphics section and then boot. Try first without inject Intel and if it doesn't work try inject Intel. Report findings.
Note: Open GL which is most of the programs in OS Sierra will not use the second gpu, but open CL programs like Luxmark will use both cards and double your scores.