I think your harp is settling in to the new tuning and environment. Is there a reason you have to have it tuned in the key of C when the levers are unengaged? Do you normally play music that requires all those sharps, especially E#, A#, & B#? A lot of harpists tune their harps to Eb because it gives them the most practical range of keys. To my knowledge, most music that is written for lever harp does not utilize those three sharps listed above, so you may want to reconsider your tuning.
Also, some models/brands of harps are just string-breakers, especially if they have gut strings in the upper range. If you love the tone this is just something you will have to get used to. For myself, the frustration of having strings break every time the weather changed or it was moved would be way too frustrating, especially when you would have to tune the replacement string 30-50 times to get it to hold. I feel pretty darn lucky that the harps I own don’t have this problem.
Well, however you decide, best of luck! And if you decide to take it back, now you have another very important question to ask