I don’t know how we could overcome the boundary of subjective first-person experience with natural language here. If it is the case that human differ fundamentally in their perception of outside reality and inside imagination, then we might simply misunderstand each others definition and descriptions of certain concepts and eventually come up with the wrong conclusions.
While it does sound dangerously close to the “is my red like your red” problem, I think there is much that can be done before you leave the issue as hopelessly subjective. Your own example of being/not being able to visualise faces suggests that there are some points on which you can compare the experiences, so such heterophenomenological approach might give some results (or, more probably, someone already researched this and the results are available somewhere :) ).
While it does sound dangerously close to the “is my red like your red” problem, I think there is much that can be done before you leave the issue as hopelessly subjective. Your own example of being/not being able to visualise faces suggests that there are some points on which you can compare the experiences, so such heterophenomenological approach might give some results (or, more probably, someone already researched this and the results are available somewhere :) ).