Hmmm. A coil is just a big amplifier/inductor. I wouldn't be worried about the existing one not working. Unless it doesn't work under load, or when it's hot out...
I use the simple $1 tools from autozone/kragen/pep-boys/Ace. They're ramp style, which means they wear, but they have an adjuster tool built into them. To use the ramp style, check the gap from the side of the arm, not the front. Or check it with the gap overlapping smaller gap sizes, not larger gap sizes.

As for the gap opening it definitely happens. My sister gave us a pickup because a shop said the trans was slipping. Nope, the engine was just hesitating because the plugs had eroded the gap from .028 to about .045. Replaced them and it ran considerably better. Desert sand in the wheel bearings did that truck no favors though xD

Happy Bricking
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1990 740 Turbo, on its way to stock specs, maybe beyond