It sure sounds like your keyboard just isn’t sending the keys, but you seem certain it is. I’d still try another, preferably wired, keyboard; as you know, you have to be certain you get all the keys down together instantly as the first chime rings; much later and you miss that critical window. And, FWIW, I’ve had numerous Macs that required you hold the combo through a total of three chimes (this was especially true on many of the G3 Power Mac series) to truly clear all the settings (you could hear the volume of the chime increase noticeably on the third chime).
It’s not as complete, but you can also use
sudo nvram -c to clear all the custom parameters (use
sudo nvram -p to list them before you clear if you want). See this page for more nvram fun:
The only other machines I’ve run into having difficulties resettings either SMC or PRAM usually required replacing the SMC controller itself, which for most people, means a new motherboard; but, even though the battery shouldn’t interfere with a PRAM reset, you can always remove the battery altogether and see what happens. Make sure it has a full 3.25v or higher or replace it with a new CR2022.
Another thing you can do to force SMC and PRAM to take another look at itself is to swap out PCIe cards, or RAM amounts. The change in these configurations forces a controller parameter review in firmware; especially if you have a card lying around that has not been installed since the last SMC or PRAM reset.
I’m still leaning on the keyboard, though; sorry if that irritates you; it’s not meant to.