It is nice that you think you can define some 'must' criteria for the Observations returned. These requirements might be reasonable, given US-Core... but what is the result when an Observation is returned that does NOT meet these requirements? I think you should change the perspective to explain that the querying agent needs to be robust to deviations, while it should expect the Observations 'would' be conformant to US-Core.
I think a note describing this would be a nice addition. That being said, we are aiming for close alignment with US-Core, not full coverage conformance.
It is nice that you think you can define some 'must' criteria for the Observations returned. These requirements might be reasonable, given US-Core... but what is the result when an Observation is returned that does NOT meet these requirements? I think you should change the perspective to explain that the querying agent needs to be robust to deviations, while it should expect the Observations 'would' be conformant to US-Core.