How do I interpret the zero-g level change vs temperature specification for this accelerometer?
According to the datasheet, nothing is guaranteed. 


You can take the delta and divide it by the temperature change and get a number, but it won't accurately represent the temperature coefficient most likely. This is common on some components and can lead to headaches if you don't understand the specsmanship games. With a complex digital device it can be even worse because there might be some mechanism to compensate that behaves badly when you look closely at it. More 'analog' parts like precision resistors and crystals usually behave more like a quadratic or cubic at worst, but often not linear. 

In many cases, the manufacturer will quote a number but buried in the documentation will explain (admit?) that it is using the "box method". In other words the drift could have a very high coefficient, say from 20 to 30°C but reverse at 30°C and go back negative as you get to 70°C. The "box" is the peak-to-peak deviation divided by the temperature change, not the peak coefficient. 

The typical figures are not guaranteed. One rule of thumb on the typical figures, such as they are, that may or may not bite you, is that most (99.9% of units) will usually be within 3\$\sigma\$.