Hi ljb,
Looks like yr getting the good oil so stick with Roff. I note you say you have the data sheet for the INA126. I find with this stuff that's all one needs.
I downloaded the ti data sheet for the INA126, and you will get lots of stuff by reading that. On p8, fig.3, there are 3 'typical arrangements'. Yours is the 'microphone, hydrophone etc'. and note the resistors to ground. To achieve a balanced input, we are talking about 'the same impedance from each terminal to ground'. To go the way Roff suggests is ok, but you need to consider that the impedance to ground is not well defined. It consists of stray capacitance and the high impedance of the input circuitry. In your case, I would go the two resistors of 410 ohm to ground method. P8 also describes the bias requirement for the device. Since the CT is operating at mains frequency, you can use some capacitance to ground from each input to attenuate the higher frequencies which will be mostly interference(probably). Your sketch doesn't show any filtering. On p4 of the INA datasheet, bottom left, is a diagram of common mode input voltage against output swing and the operating region shown is the region in which the common mode performance will be satisfactory. However, the degree of balance of the input wiring and circuit will probably dominate the common mode performance.
Your arrangement only needs to be satisfactory, but the fine tuning can make a huge improvement in the end result.