Personally I do not think that Sasha can be Summer because I cannot believe that Summer would stand by and watch the MC suffering without reaching out to him and trying to lessen his distress. Nor do I believe that the MC would fail to recognise the "love of his life" so easily if he saw her, even from a distance and her outward appearance had changed. If Sasha actually is Summer for her to keep her distance and silence either the MC must have done something really terrible to merit such exclusion, or she must consider that she herself represents a potential danger or threat to him, for unknown reasons, and is staying away from him deliberately in a bid to ensure his safety.
Speculation on my part of course and, more than likely, completely and utterly wrong.
But what the heck?!