The downside of references is that it doesn't tell you how much the references know-- you can get a good reference from someone who had a good time.
I don't think any amount of education will make a good instructor.
The NRA's certification system reminds me of a Ponzi scheme without the benefit for first tier investments.
One of the drawbacks of a class-based instructor education system is that it has the capacity to produce firearms instructors who can't shoot.
For my money, I'd say that firearms instruction is a skill and a trade, and can only reasonably be taught as such.
So I'd advocate an apprenticeship, and I'd say four to five years.
"Huh?" --Jammer Six, 1998