They do absolutely zero compliance testing
I'm not sure what you mean by that...

It also seems like if they don't want to pass a manufacturer's H & N restraint - particularly if the manufacturer isn't a member of SFI - then the SFI standard is seemingly revised just enough to exclude that product. Coincidence? ...
SFI standard 38.1 for head and neck restraints has never been revised since its creation. I've seen that there's a lot of people who feel there is a conspiracy behind that standard. For example, the fact that the G-Force SRS failed by just 10N. But that truely is a coincidence. When Neck Tension is >4000N, that is the point where injury can occur. That number is an international safety standard, it was not determined by the SFI. G-Force scored 4010N, therefore it simply doesn't provide effective protection. No conspiracy there, just facts.
Some people feel the Isaac is unfairly excluded also. It meets all the safety standards, but fails because it does not have a single point of release. That particular rule might be questionable, but personally I agree with it.
This standard certainly isn't perfect, mainly because the technology is new and everyone is still learning. In any case, between the current FIA and SFI standards for head and neck restraints. I still think SFI does a better job.
Since no sanctioning body has the ability to allow or disallow products on a case-by-case basis. They must rely on one of the existing standards.