Can you increase data rate by increasing number of bits?
Theoretically you can, but in reality you can not. The problem in raising the number of bits one symbol represents is that you have to space the symbol levels closer and closer together. Therefore even small amounts of random noise lead to misinterpreted symbols. A first attempt at calculating the maximum data rate is Nyquist's formula:

maximum data rate [bit/s] = $2H\cdot log_{2}(V)$

Where $H$ is the bandwidth in $Hz$ and $V$ is the number of symbol values.

This formula indicates that unlimited data rate can be achieved when enough symbol levels are used, but, as we have already seen, that is not possible because of random noise. Shannon's formula takes that into account:

maximum data rate [bit/s] = $H\cdot log_{2}(1 + S/N)$

Where $S$ is the signal strength and $N$ is the noise level.