The editor will pick up the relevant selector that applies to the element you are changing.
Do you mean that the actual DIV ID's and Classnames can change dynamically? Can you provide some more detail on this? If that is the case, it could be promlematic. You will need to make sure that you are using selectors that are not going to change during the experiment.
Would you mind posting a URL here so we can take a look at the HTML structure of a SquareSpace page?
I've worked with some sites that are using Squarespace. They could be hard to change and the ID names can easily change as they're using some unique IDs that can change on every edit. But in most cases the class names remain the same in my experience. You would have to probably create longer XPaths to make sure you'll build in some extra security so a test won't possibly screw up your lay-out.