Sounds to me like a few problems there.
If you tested for, and got a clean break, then your rennet is fine.
The curd 'shattering' when stirred is typical of homogenised milk.
I would not assume that 2 week-old-sitting-in-the fridge kefir is active enough for a starter. To quote from Priscilla, Queen of the desert:"Assumption, (Bernard), is the mother of all ef ups".
Welcome to the trial-and-error learning curve