Update:
It seems the method used (NewPointOnEdge) requires actually having a solid. Even though an adaptive can host on a model line that is part of a family within the project environment, the API method only works for edges (meaning you need to have a solid first to get edges from). If using just lines in a loaded family - the adaptive will place itself within the project at the global coordinates that correspond to the local coordinates within the family of the points along the line you are trying to host on. If you model a solid within the family such that one edge falls exactly along the line you wanted to use, then modify the code to find the edge rather than the line - everything works as expected. Bottom line - I think this is a case of that which works in Revit UI does not always work in Revit API.
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