That's a NICE one!
Well, before I can post my theory I need answers...
How old was Sharon in episode 286-288 when they were in New York?
Well, let's say...if she took APTX 4869, then she would become ten years younger. And then Sharon said that she haven't seen Chris for ten years. That means when Chris "appeared", Sharon already took the drug. Then in the NY case, that would be ten years later, Sharon(who already took the drug ten years before) became 10 years older, back to her age when she took the drug. And Chris, at that time, was never heard. So, Sharon took APTX 4869 again then acted as Chris (that's the time when Sharon "died") until now. I'm not really sure, but it may be wrong or what.