Magnetos---best way to check

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There's two types of magentos checks.

When checking the magentos during the runup (briefly to L, both, R, both) what you are checking is that all spark plugs are functioning fully. You ground the R circuit so the engine only runs on the L magneto. If any of the magnets, or leads, or spark plugs is in the L circuitry is duff, you are not firing on all cylinders (literally) so you lose 1/4 or 1/6 of the performance of the engine (depending on the number of cylinders of course) and this translates into a massive RPM loss and rough running (trust me, you'll notice: I had this on my first solo). In contrast, if everything is allright the only thing that happens is that the flame front is ignited slower so the RPM drop is small.

When doing a dead-cut check you're not checking the leads, spark plugs and everything, but you are specifically checking that the grounding circuit is in place for all magnets. If you were to do this one magneto at a time (L, both, R, both) a normal drop in RPM only suggests that some of the grounding circuitry is in place but you cannot be sure that all circuitry works as designed. Think about it: suppose you switch to L but the grounding circuit for one of the plugs in R is still active. Three cylinders are now running on the L spark plug alone and the fourth is still running on L+R. All cylinders are firing and you are not going to be able to distinguish this RPM drop from what it should have been. But when you switch the magnetos to "Off" you are still left with one live spark plug in R.

Now my main question is: how likely/possible is it that the grounding circuit fails partially. In other words: that some of the spark plugs are alive in a certain grounded circuit, and other are not? If the only failure mode of a magneto is a complete, 100% failure, then of course you can detect whether the grounding circuit works by switching L/Both/R/Both. But if you have a magneto system (or electronic ignition, or whatever) that can fail in such a way that some spark plugs in a circuit are left "live" while others are grounded, then a dead-cut check is the only way to find out.

Disadvantage of a dead cut check is of course that you blow unburned fuel into the (potentially hot) exhaust (+ turbo if you have one) and eventually in the atmosphere. This might be dangerous depending on exhaust design and how hot the exhaust really is. The other disadvantage is that you could do the dead-cut test too long and thereby stop the engine, with air/fuel mixture still in the cylinders. Maybe not the way it was designed, although for instance the Rotax 912/914 is intended to be stopped by grounding the magnetos, instead of idle-cut-off.

What surprises me is that the authorities are playing a role in this. Isn't this something that's simply written in the POH? A dead-cut test might be completely sensible for an engine with a direct drive, heavy non-adjustable prop (lots of momentum), no turbo, traditional magnetos and straight pipes as exhaust, but completely inappropriate for a geared drive engine with a light CS prop, turbo, electronic ignition and a silencing exhaust system.