gah, silly me.
you know when you look at something for so long you can not find what you are looking for.anyhow, what i was looking for was pretty much right in front of me, but after making some food and having a break and some ghost in the shell, it just hit me. was a simple change of 7 to 15.
Its always good to move all your important parameters out of the body of the code and make them either #defines or parameters to a function, so that you can change them easily and so you change all the related ones in step. Try to always separate code from data, then the code is general purpose and contains fewer assumptions. All unchecked assumptions are bugs waiting to happen.
You might want to check you are calling pinMode for pins 5..7, not just 2..4, and rewriting the code as a single function that takes a channel number and returns an ADC value might be neater. You can then hide the details of addressing the multiplexors and choosing the analog pin within that one place.