1. Instrumentation amplifiers reference signals to signal ground (GND) normally. But you also couple Vref into the inputs. This is unusual and as long as I don't know the actual reason for that, I cannot say whether it's good or bad.

2. The LF412, like many other OPamps, needs a biploar supply, when it's intended to handle potentials near 0V at the inputs. The specified input voltage range of this OPamp does not allow what you intend: Your input voltage is only about 0.5V higher than the negative supply voltage, which will not work.

3. But even if you take a bipolar supply, the gain isn't properly adjusted. When R5 is 1k, then the first stage has a gain of 19. But R11...R14 form an additional voltage divider, so V1 is amplified by a factor of 9.5 in the first stage.

Kai