For the first one, the table is basically showing that for the mutant, every incorporation of C occurs with a frequency of 10^1 with respect to 10^-2 of A. In other words, 10 C to 0.01 A. Then it just becomes a ratio problem as they say.
For the second one, why do you think the nucleotide composition would change? You're assuming simply that a G has been replaced with oxo-G. That does not disrupt the hydrogen-bonding network.