Let's go with this. Every domestic violence case may turn violent - and, again just for the sake of argument, we'll assume that this requires the use of deadly force by the police, so we arm them with deadly weapons.

If those cases are extremely rare, and stats I've seen show they are, there is still no need for the weapon in the other cases. We can go ahead and disarm the other officers, and have a small portion maintain arms for potentially violent cases in your given scenario

Of course, I'd argue that you don't need deadly force even to deal with the majority of potentially violent crimes, but that's a separate matter.

This is like a turkey problem -- every day is a jolly day until thanksgiving comes, once a year.

DV cases are rare? On what planet? Police deal with DV cases continually.

Relatively rare.