I agree with most of your observations. We also have been hired as course marshals for some races. One of our largest clients has a "Technical Director" who is the course marshal for all their races. We work very closely with him before and during measurement and he is present at the races. The certifier has never (to my knowledge) had any role in checking the course layouts on race day.
As to the magnitude at which the certifier becomes involved, I believe that would be any USATF (National) Championship race. There are rules about a qualified validator (not necessarily the certifier) being on the course (in the lead vehicle) on race day. Mike Wickiser is the guru (Validation Chair). This is nicely covered on the "Verification" page of the "Resources for Event Directors" page on the USATF website.
These championship races are a tiny fraction of the races held on USATF certified courses. The vast majority rely on the integrity and professionalism of the race director to insure that the racecourse is the course that was measured and USATF certified.