How to cite paper when there is a gap between conference date and publication date
What matters is the publication date, not the date of the conference. I don't know APA well enough, but in my field, the format would look like this:

Wolfgang Bangerth, "What is reality?", Proceedings of the 12th 
conference on impossible research, South Pole, March 12-24, 2010,
A. Stnas (ed.), Springer 2016.


In other words, the date of the conference appears as part of the "Proceedings of ..." text, whereas the date of publication (2016) is the normative date of publication.