Having an open purge valve brings in fuel vapors, so the mixture would get richer. The O2 sensors would read that and the PCM would adjust. I forget -- how old are the O2 sensors?
I'm curious if you have a physical alcohol sensor. Either way, it would be interesting to:
1) reset the LTFTs and monitor them over time
2) read the current fuel alcohol content as determined by the vehicle, using an advanced scanner
3) compare that to the fuel you've been buying
3a) if they don't compare favorably, reset the alcohol content
4) sometimes there are revised PCM "calibrations" that can be updated; a shop with access to GM programming could read yours and advise if an update is available. There may be one that updates the alcohol content algorithm.
You could also consider using ethanol-free fuel for a few tankfuls, as a test, and if it's available in your area. Monitor the trims using that fuel.
The purge valve will pull vapors initially but soon should be almost "clean" air as the charcoal "dries" out. That's my understanding but I haven't seen any data on that.
My 4 O2 sensors are new, MAF is cleaned but not assumed to read correctly but does show expected data. The air filter was only slightly off color, no debris. Connections have been triple checked but there may be a crack somewhere.
There was an explanation regarding alcohol sensors. Apparently there used to be a sensor and then they did away with the sensor and used the O2 sensors and a few tricks to estimate the alcohol content. I don't know if this was for flex fuel or the "regular" fuel versions. I think there is also an alcohol reset but I don't know if my scan tool can do that - after I get it running again.
Additional info that I failed to post. During idle - I only have about 4-5% LTFTs - which supports my hypothesis. I would need to double check or unplug my purge valve to confirm to some extent.
A theory (matches what I see on my Tahoe):
Idling - only the alcohol adjustment is in play (LTFTs are + 4 to 6%)
Normal Loads - 10% alcohol offset and purge valve are in play (LTFTs are + 9 to 12%)