So far, this seems like the most 'safe' outcome for Celestia. The guilt level would be a lot lower but definitely still there.

However, now that I think of it, Celestia has always kind of enjoyed 'testing' Twilight through putting her in a position of power as she kind of 'advised'. So this outcome may not cause Celestia any guilt as much as it would give her what she always wanted, no worries about her guard, and be justified as just another 'test' for Twilight. After all, twilight did look up to Celestia and why wouldn't she approve of sharing in what her mentor always enjoyed in, too? It makes sense, right? Personal student who is one not just for talent but shared interest? Win-Win, right?