Maybe somebody has a suggestion about such questions? 1. What are these strange spikes at the beginning of each clock cycle? 2. Why SDA data ground level have different values?
SDA has different low levels depending on whether the master or slave is pulling it down. This is actually a useful debug tool because you can tell who is controlling the SDA at that moment. You will probably eventually wish to determine why they are pulling down to different grounds, though.
The spikes are overshoot, caused by impedance mismatch.