(Original anime spoilers) Questions about the rounds

Recently I rewatched the original Madoka magica anime and I was confused on certain aspects of the timelines we saw

1. why was Madoka already a magical girl before Homura arrives in round 2 but in rounds 3 and after she isn’t and only becomes a magical girl later on in those timelines.

2. why doesn’t mami know about walpurgisnaught in the later timelines

3. was sayaka a magical girl in round 1? Or was that only something that became a thing in later timelines

Sorry for the weird questions, ill give extra clarification if needed