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I performed magnaflux inspections for a number of years and demagnitised parts afterward using an AC coil. Permanent magnets will lose gauss after a time but some residual remains for many years. With the type of magnets a model T has, they should remain with good enough gauss levels to last at least a decade or two.
The 1919 through 1927 model T, had a power wire that connect from the battery to the switch. A circuit won't flow current till it has a pathway back to the ground source. There may be a problem with the switch but it could be checked with an Ohm meter for continuity to ground. A test light can be used but it has to be isolated and then power applied to one side to safely use the test light on the other side. The magneto was used to power headlights in some early model T cars but they used a high voltage bulb to do that. A regular bulb would likely burn out. The Mag terminal on the switch is either open or it's connected to the coil box to function the trembler coils. The switch should be tested independently after isolating each terminal that is applicable to it's respective circuit. Light circuits should not be involved with magneto function. When the switch is on battery, it has a common connection with anything else that works off the battery. Look at a photo of a switch repair kit and you can see that ignition circuits have a separate contactor on the rotating plate.