I was considering the merit of setting an exam - if the "student" passes he is good to go, how he achieved the standard is irrelevant. This could be applied to any walk of life and any exam. If a lawyer were to pass the bar exams then he should be good to practice.
I remember a talk from a long retired CAA CFE. He described the procedure of old, where an applicant for an IR would turn up and hire the CAA DH Dove on which the test was to be conducted and pay his fee for the test. He would undoubtedly fail first time, but the examiner would give him useful corrective tips through the test. The next time, he would perform better, but probably still fail. After a half dozen such "tests" the applicant would get quite good, and might well pass. Not ideal, as you point out, hence the introduction of the requirement for mandatory training, which is now at the other extreme.
Bear in mind that the ICAO IR requires only 10 hours of instrument training. It does however, require 40 hours of total instrument experience. It seems unlikely that 10 hours of training would be sufficient for most to pass an IR test, thus the practical minimum training is likely to be 20-25. I doubt many would disagree.
With the IF from the PPL, that leaves about 10 hours of IF to be acquired before an IR can be issued. Whether or not that requirement for extra experience has a safety case is debatable -- I think the stats you quote for the IMC rating are impressive, but are based on sufficiently small accident rates that it's difficult to reach significance. The 40 hours is however, an ICAO standard, and for the sake of such standards, I think the cost of accepting an extra few hours of IF is outweighed by the benefit.
So how does the candidate acquire such experience? One option is mentoring, though that has the issue of who is in command, the mentor or the mentee. Another is that the candidate could be permitted to pick up useful instrument experience in a relatively benign environment which wouldn't require the hardest bits that the IR demands. For example, you might permit the candidate to fly in IMC enroute but avoid instrument approaches...