Two points.
1. When the trottle body is properly set up the engine should idle perfectly normal all day at 700 rpm with idle air motor disabled at the test jumper point. When the idle air circuit is enabled idle will then rise to 750. It is likely your throttle body needs cleaning and adjustment as well.

2. Ignition pulses are fed to the coil primary negative terminal. The tach signal is tapped off at this point. If tach reads 0 you would have no ignition. Of course a problem with high voltage components would not affect the tach. The adage is therefore true if restated as "no tach, no ignition". If a stall occurs at driving speed it takes a while for the engine to wind down as road speed decreases. If tach plunges to 0 before engine does then you have lost ignition pulses to the coil. Observing tach is a good diagnostic tool. You can even observe it during a no start. If it nudges up slightly while cranking then you do have ignition pulses at the coil.
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David Hunter