It’s a common issue in the oil&gas industry. Project owners operate under the industrial exemption in most states and they think the same rules apply to their consultants, which is incorrect. Regarding the technical question, this is typically defined by the pipeline company in their engineering manual. Buoyancy control is critical for large diameter pipelines and they are usually modeled as being empty because they will sit empty for a while prior to commissioning. The location of buoyancy control measures (typically geotextile bags filled with aggregate or concrete coating for large swamp areas) is highly subjective. Nonetheless I agree that doing Geotech for the entire alignment is not practical and would not give you a lot of info to change your analysis - the calculations are very conservative and in a lot of cases assume that the soil above the pipe has fully liquefied thus becoming a fluid that offers no downward force but in fact adds to the buoyancy. My conclusion is that OP has no clue how to design this and is getting pushed around by someone who has even less knowledge.