I’ve never encountered the second usage (a fact which may point to others having a similar view of its conflict with the primary definition), but it would seem that one would select between them based on the context in which either was used. Alternately, consider ways in which they may not in fact contradict one another, but both suggest a single quality that might be appreciably illuminating in some cases and nonetheless insufficiently conclusive in others.
For sake of comparison, let’s look at these uses of the word “scenic”:
- Our trip to the library would have been more educational had you not taken such a scenic route to it.
- Thanks to our driver’s preference for the scenic route, we got to see much more of the countryside.
In both instances, the word is used to describe a chosen route (this is commonly the case for “scenic”).
In the first, the speaker suggests that the purpose of the trip was to read books in a library. Fulfillment of this intention was upsettingly hindered by someone’s selection of a meandering route, a route that added travel time that could have otherwise been spent soaking up the knowledge of history’s authors.
In the second case, the situation is different. The speaker implies a desire for maximum exposure to the surrounding area, whatever vistas and encounters that may lead to. This goal is pleasingly met by the driver (possibly the same person being addressed in statement #1!), despite the fact that the selected routes once again took passengers all over the place.
Returning to the word in (your) question, it seems possible that a single description might be described as circumstantial in either sense of the word. It could, for instance, illuminate in full detail the scene on the street some fateful day: the colors of the flora, the sounds of traffic, even a detailed description of some footprints formed in the mud that lay on the path of the person providing the testimony. Truly circumstantial(2) regarding every nuance of the scene. Even then, however, such an account would only suggest the guilt of a person in muddy boots discovered later on the other side of these footprints from a crime scene. Presented in a trial, this evidence would again be described as circumstantial(1) in that it does not prove that the muddy-booted person committed the crime (or even left the footprints).
A description or other evidence of circumstances, no matter how complete (circumstantial(2)), is denigrated in the context of a trial as mere suggestion via context (circumstantial(1)).