Wheatstone circuit not functioning
Are you sure the dummy strain gauges don't see any strain? You say it works with three 120\$\Omega\$ resistors instead of the dummy strain gauges. If the dummies don't see any strain, why are you using them in the first place? In a Wheatstone bridge you would either have one variable element combined with fixed resistors, or 2 or 4 strain gauges which work opposite to each other, so that the measured signal is doubled.  

That horrible thing that you call a schematic looks OK, and your gain of 6000 should probably be enough to get some signal. Note that at a gain of 6000 less than a mV difference in the InAmp's input is enough to send your output all the way to ground or \$V_{DD}\$. You might need less gain.  

I would try again with the 120\$\Omega\$ resistors.