I need one word that describes something as both (1) necessary/essential and (2) not sufficient/non-comprehensive/lacking
Solution 1:
The item you are describing is prerequisite for whatever more comprehensive thing will be based on it. The term prerequisite certainly indicates that your subject is required or necessary, but also implies that its establishment or attainment is not the main goal. The noun form, referring to your subject as "a prerequisite" for something else, may be clearer to your readers. The adjective form, while less commonly used in my experience, is nevertheless also correct.
Update: Now that you've added the context, I'm not sure if the word prerequisite is appropriate for the ideological positions you're addressing. I'm afraid this comparison between moralities is over my head. You may wish to say "fundamental, yet limited," instead of looking for a single word... or just "foundational".
Solution 2:
Elementary may suggest the idea of something basic but not definitive:
Of, relating to, or constituting the basic, essential, or fundamental part: an elementary need for love and nurturing.
Of, relating to, or involving the fundamental or simplest aspects of a subject.
The Free Dictionary