Nodal analysis: how can I represent the current i2 in terms of voltage over R2?
The problem here is, how can I declare i2 in terms of voltages and
  resistors? If the current source was not there I would say \$i_2 = V_c/R_2 \$
  but with the current source there I don't now.


The problem is, i6 doesn't really exist, C is the same node as the both the nodes on either side of the current you have drawn for i6. Since the voltage for node C is the same as the node between R2 and the current source, the resistance for i6 would be zero and technically the current would be infinite.

A better node equation for node C would be this:
\$0 = i_1 -i_2 -i_3 + i_7\$
And you need to write C on the node between R2 and the current source

The voltage equations for R2 is \$i_2 = V_c/R_2 \$ so you are correct, you can take any resistor and the two nodes around it and find the current through the resistor

\$ \frac{V_A- V_B}{R} = i_R\$

In this case Va would be Node C and the other side is ground, which you have correctly identified.