You were right on the problem. Thanks for your help.
I am able to boot into the welcome screen, and passed all pages subsequently.
Just to help another Asrock user,
- Disable all LAN, Audio, Serial
- Disable Onboard Graphics
First Time Boot (Welcome screen)
- At Internet Connection page, select Mac is not connected to Internet
- Do not enable any internet services
Intel HD Graphics 2500 was the key issue in all of these.
- NullCPUPower is required
- FixUSB Ownership needs to be true in Clover.plist
I've successfully booted into El Capitan. Now shutdown and go back to your BIOS and turn on the devices you need (e.g., LAN / Audio). And take your time to fix the rest.