I mean, I could see getting behind an argument that Lily wasn't the Summer Knight when she was a statue, because, well, she was a statue. But she had to have been the Knight before (and after) she was the statue, or there wouldn't have been an imbalance.
I guess that works... She was SK for a moment, then she was a statue (taking the knight-mantle out of play, hence imbalance), then she was SK for another moment, then she was SL.
The SK mantle was there all along, but it was hidden... REALLY hidden. If Titania cannot even FIND her Knight / mantle -- to which she is mystically attuned/attached -- it says something about whether said "Knight" is in fact BEING a Knight. Hence, imbalance.
Still... From the POV where it was originally raised (the passing of a Mantle) I guess the SK mantle DOES pass to Lily during the course of the books (just before the start of SK (the novel)). I'm still disinclude this one from any attempt to draw conclusions -- I don't think the books give any details of Lily's "Knighting" but given Aurora arranged it under influence of N'fection, I suspect it isn't a datum from which to generalize about how things do/should work.