Genuine question, how accurate are Tazers? If the partner was in a headlock, was there any risk of tazering the wrong person while the gum was more accurate?

Speaking independently of the story- Not very by comparison to firearms. Something like 50% less accurate. There's also the issue that tasers will not always incapacitate someone. That's a gamble if someone has a weapon and the range to use it.

Part of the rationale in using a firearm is the need to body someone before they can use a weapon where non lethal methods are just not as effective.

Of course, when you investigate yourself you will always find that you used your firearm in the appropriate situational context.