But I think then he/it wouldn't quite be Data, unless he was copyable, and that is a side of his existence as a computer program that seems to have never been addressed in lit.
This is a bit subtle I suppose, but I actually wasn't saying that Data is copyable any more than we are, strictly speaking.
Christopher pointed out quite correctly that we are more than our memories: we are the exact state of our physical body from moment to moment. The problem thus is not merely to copy Data's memory around, but to recreate the state of his neural network as formed by the steady succession of his life experiences.
The potential for recreating this state lies in the fact that Data may record his life experiences with greater fidelity than humans do. Human memory is quite lossy. Being a machine, however, and assuming sufficient storage, Data may be keeping logs of all of his sensory inputs: What his eyes are seeing, what his ears are hearing, ... as well as less obvious forms of sensory inputs like the position and condition of his limbs. This is unlikely to be Data's
only form of memory since it would be very inefficient to search - at the very least he would need to build additional indices -, but it would have obvious advantages for optional recall.
All of these inputs reach Data's neural network in a particular way and format - simplified, a wire sticking into it. If you can create an "empty" neural network with the exact same state as Data's before his initial activation - i.e. have the same exact starting conditions - then feeding the logs of those inputs into this neural network over the wire gets you a long way toward accurately recreating the state of his neural network at the time of his death.
A remaining major problem is things the logs could not cover, like physical circumstances that were affecting his neural network at the time it was processing those inputs (i.e. the radiation field example). You might be able to explain this away by saying Data's cranium is extremely well shielded and that you can count the number of events that did affect his brain on one hand and simulate them.
Another problem is that it might take very long because the replay might have to be done in realtime (i.e. it would take exactly as long as Data had lived) to allow for the time the physical processes inside his neural network hardware need to happen in response to each input. How much you can compress time there depends on the exact nature of the hardware.
Then I guess what the script suggests happen is that B4 be developed, not (a new) Data?
Well, I assume that Data would have seen resetting B-4 and creating a copy of himself in place of B-4's own mind as the murder it would be.
I think to resurrect Data you do need to first build a new body. B-4 is important because he may have backup of Data's sensory input logs.