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The augmented leads (aVR, aVL, and aVF) require a minimum of three electrodes (multiple electrodes are connected to ground).

An electrode on each arm gives you Lead I.

Lead I is not as useful as Lead II, but it's still plenty useful for diagnosing all sorts of cardiac issues (there's a lot more that can go wrong with your heart than just an MI).

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Presumably a single lead would still be able to detect certain arrhythmias? I.e. if there are no QRS complexes, then there may be something wrong with the person wearing the device. There are benign explanations for why the signal might be weak but also lethal causes like Vfib -- with a few additional, I don't think it's implausible for a device to be able to make the decision to ask the user "hey, are you okay?" and then potentially seek help if the user doesn't confirm that they are OK within some set period.

Absolutely. Consider the fact that an AED only has one lead to work with, and it is able to recognize a couple different lethal arrhythmias in order to deliver a shock (and more importantly, _not_ deliver a shock at any other time).

To be clear, by "lead" here, I mean a pair of electrodes. A single electrode (like you might put on a watch) is useless.

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