Well I've got a C8 and four batteries sitting here all at 3.5V...however no charger yet, so that's on its way along with another C8 for the handlebars. Waiting for a charger to turn up when you have a new torch sitting there is making me squirm somewhat!
As for multimeters, I got a very cheap one on a deal from Screwfix several years ago that is still going strong. In case anyone doesn't know how to test a 18650 battery voltage, here's an artist's impression!
(Except I normally lay the test pins flat, not sure if this is necessary or even correct).
Regarding how to test it under load, that's beyond me I'm afraid but I'm sure someone can advise.
Regarding monitoring when you batteries are reaching 3.5V, personally I'm just going to recharge them pretty often as once you've gone to the faff of getting them out of the torch, you might as well just stick them in the charger (unless you need them of course).
Final point; I'm confused as to why the C8 only pulls 1.15A on high? Seems low, maybe not.