Academic integrity question on self-referencing and changing a paper after acceptance
By my understanding:* You've presented some work-in-progress at a workshop, without proceedings. You have a conference paper accepted and you've been invited to make some minor revisions. The workshop presentation is built upon data used by the conference paper, but the methodologies are different. The workshop presentation drew a conclusion that you're contemplating including in the conference paper, albeit, that would change the narrative a bit. You feel that you made a mistake by not [citing the workshop presentation] in the conference paper. You're also concerned that not citing the workshop presentation would mean that you pretend that the improvement is bigger than it is, albeit, you're concerned that adding a new citation, which reviewers haven't seen, is very unethical. You're considering building upon your workshop presentation in the future and publishing that work later (and you consider this as the proper solution).

My thoughts: 


You are under no obligation to cite your works-in-progress, especially those that are unpublished. That said, you may choose to. 
You should not extend your conference paper to include new ideas - e.g., the conclusion you drew in your workshop presentation - since they haven't been peer-reviewed. 
You could later publish your work-in-progress with citations to your prior conference paper. Alternatively, you could combine that work-in-progress with your conference paper to derive a journal article, the former aspect would be considered a new contribution (since it hasn't been published previously). In both cases, you could - if you wanted - cite your workshop presentation, since it presented some early results.




*I've summarised my understanding, since I'm not sure I've correctly understood the details presented by the OP.