Output impedance imbalance and CM noise in truly floating sources
For scenario 1 just replace the voltage sources with 0 volts i.e. forget about your wanted signal  - just imagine your signal is 0 volts. Then imagine an interference source of (say) 1 MHz of which one side is grounded and the other side connects to line A via a 1 nF capacitor and line B also via a 1 nF capacitor. Then ask yourself which line is going to show signs of interference; the line that is in-effect grounded (line A), or the line that has 1 kohm to ground (line A)?

For scenario 2 use the same idea and note that line A looks like 10 kohm to ground whereas line B looks like 11 kohm to ground. Which line will receive more interference?

I'd set up my simulator to look at the problem this way: -