Your argument that P2 is question-begging misses the point. The assumption is not that the replacement neurons can replicate consciousness, it is that they can replicate the behaviour of the biological neurons. This nuance is missed if words such as "function" are used: perhaps function includes consciousness, and then the assumption would be question-begging. But for the reductio to go through, the assumption is that what is replicated is just the behaviour without consciousness. So the artificial neuron will accept input from the biological neurons, do computations, and send output to the biological neurons such that those neurons behave in exactly the same manner as they would have if the output had come from the original neurons that the artificial neuron replaced. You might say, using the Biology Argument, that neurons contain something special such that their output (the nature and timing of chemical and electrical stimuli) cannot be replicated by silicon chips, and therefore the behaviour of downstream neurons will be different, and therefore the behaviour of the subject will be different. But the thought experiment can easily be modified by saying that the artificial neuron contains some alien technology rather than silicon chips. This alien technology reproduces the behaviour of the neurons, but not consciousness. The argument is then that if we could, by any means, separate the behaviour of neurons from consciousness, we would be able to create partial zombies. If you agree that partial zombies are absurd, then the argument goes through that it is impossible to separate the behaviour of neurons from consciousness; or equivalently, if we could find a way to replicate the behaviour of neurons, we would necessarily also replicate the consciousness.
Your argument that P2 is question-begging misses the point. The assumption is not that the replacement neurons can replicate consciousness, it is that they can replicate the behaviour of the biological neurons. This nuance is missed if words such as "function" are used: perhaps function includes consciousness, and then the assumption would be question-begging. But for the reductio to go through, the assumption is that what is replicated is just the behaviour without consciousness. So the artificial neuron will accept input from the biological neurons, do computations, and send output to the biological neurons such that those neurons behave in exactly the same manner as they would have if the output had come from the original neurons that the artificial neuron replaced. You might say, using the Biology Argument, that neurons contain something special such that their output (the nature and timing of chemical and electrical stimuli) cannot be replicated by silicon chips, and therefore the behaviour of downstream neurons will be different, and therefore the behaviour of the subject will be different. But the thought experiment can easily be modified by saying that the artificial neuron contains some alien technology rather than silicon chips. This alien technology reproduces the behaviour of the neurons, but not consciousness. The argument is then that if we could, by any means, separate the behaviour of neurons from consciousness, we would be able to create partial zombies. If you agree that partial zombies are absurd, then the argument goes through that it is impossible to separate the behaviour of neurons from consciousness; or equivalently, if we could find a way to replicate the behaviour of neurons, we would necessarily also replicate the consciousness.