After reading though the latest you have mentioned, it does seem that you have some conflicts in your setup.
First though, Gigamaxx's advice is good, but note: Xcode is free, you can get it from the App store. It only costs various amounts of money if you want a commercial developer's license. The part you would be using is the included plist editor. Xcode is a large download though, it has an largish installer which in turn downloads more largish files.
There are other alternatives, you can mount the EFI partition and then use a text editor to edit the config.plist file -- TextWrangler (free from BareBones) is excellent for this, Apple's included TextEdit is also good, but be careful with it as you must make sure things like smart quotes, etc., are turned off or you can get weirdness in your files. Also, if you are inclined you can use the Terminal app (in your Utilities folder) to mount the partition and edit the files and be reasonably confident there will be no oddness.
Back to what you are doing. Since you have said you added in the patch for the Safari issues, and are getting the Intel Iris Pro screen, it seems that you have inject intel set to true in your config.plist file. Pulling the RX480 card and having things work is because the OS is using the patched HD4600 accelerator and not trying to mix the cards. if you take the patch off, it is a 'good bet' that you will suddenly start having some issues with Safari, maybe YouTube, and FCPX 10.3.
Once you mount the EFI partition on your desktop, open it and you should see a folder called EFI and a file called boot. Open the EFI folder and you should have three more folders called APPLE, BOOT and CLOVER. Open CLOVER and there will be a lot of folders inside. Ignore those for the time being. There will also be a file called config.plist. This is the one you want to look at.
From here, the specifics for your card will be different than for my card, but some things are the same. Scroll down to the Graphics section:
Code:
<key>Graphics</key>
<dict>
<key>Inject</key>
<dict>
<key>ATI</key>
<true/>
<key>Intel</key>
<false/>
<key>NVidia</key>
<false/>
</dict>
</dict>
Also check the SMBIOS section and make sure you have something like:
Code:
<key>ProductName</key>
<string>iMac14,2</string>
where your product type is a Macintosh product that has a non-igpu graphics card (i.e. if you are using the iMac 14 series, use iMac 14,2 NOT iMac 14,1). If you are using the 15 or 17 or NN series, that still have to be one with the 'extra' graphics card.
Once these pieces are configured, the parts Gigamaxx is telling you about fake ID and whatnot for your RX 480 card will need to be taken into account. I believe that once you have this all dealt with, you will be able to go back and remove the Safari patch and it will still work with those videos.