How can I be sure that all variables are still active?
You can't. The plugin doesn't explicitly track whether a particular API construct changes. The purpose of the plugin is simply to take an external API source and make it available in Indigo. That is by design (and why it's called
GhostXML).
APIs really shouldn't change very much in ways that will break things. Good practice demands that source APIs be static--new data points are just fine of course, but existing ones shouldn't really change that much--if at all. To change an existing API data source means that the provider runs the risk of breaking everything that uses its data. Not a good idea. For APIs that are truly dynamic, I wouldn't recommend linking directly to custom GhostXML device at all. I'd recommend using a Python script (that you control) that suits your needs specifically. In that script you can pick and choose what you want and control for missing data points.
API Source --> GhostXML Plugin --> Your Script --> Indigo Processing