Pads very close together, no space for guard ring? What should I do?
The output of the instrumentation amplifier is low impedance, guarding should not be necessary for A13.

Any leakage currents at that node will have an insignificant effect as the negative feedback within the instrumentation amplifier will avoid any change of the voltage.

You don't state the value of the resistance in the RC filter between the two amplifiers but to minimize leakage effects I would not recommend more than a few kilohm.

You don't indicate the part number of the instrumentation amplifier.