LED strange behavior
There are a couple of things wrong with the circuit.

If you need the input of the circuit to go up to (or beyond 8V) it would be better to divide the input voltage down to say a range of 0-5V.

Currently, when you get to 8 or 9V the emitter of Q1 is forced to the same voltage and there is no voltage to drive the LED. This is probably why the LED is getting dimmer.
Put a resistor divider between op-amp 1 and the input of opamp 2. Say two 4.7k resistors. You will need to reduce the value of R2 by the same amount to say 100 ohms.

As shown you will be overloading the output of the first op-amp by shorting it's output to ground with the transistor Q2. The resistive divider from part 1 will fix that as well as Q2 will only have to pass the current through the resistive divider, 1-2mA.

What is the design range of the input voltage?
To ensure the LED goes off when the input is at zero the op-amps need to be ones that ground is a legitimate value.
Q2 will just limit the current linearly when it starts conducting, it will not oscillate on and off.