This looks like an interesting and fairly strong argument. But what sort of mechanisms are proposed by the YEC crowd for their accelerated nuclear decay? The detailed mechanism might affect this argument. E.g., if one supposes changing the decay simply through some sort of compression/expansion of time (Gerald Schroeder's view comes to mind), this shouldn't change the decay energies. But I doubt that many YEC folks are proposing this--most don't seem to like Schroeder's positions, and it would make God look deceptive.
Chaffin (of the RATE team) seems to suggest that the strong nuclear forces (or maybe the electrostatic force) may have changed. But a change in the strong nuclear force would affect the halo radius, as you point out. A change in the electrostatic force may not affect halo radius, but would probably have huge (disastrous) effects on all of chemistry. A change in either of these fundamental forces would also likely destroy the delicate balances which are required for life (as pointed out in anthropic principle arguments).
It sounds like someone else in this thread is proposing a stimulated decay process (by some as yet unknown mechanism)? Perhaps that would get around your argument?
Your argument may not be foolproof, but it seems to be a strong argument that a change in the strong nuclear force is not a possible mechanism for accelerated nuclear decay. The decay rate would have had to change by roughly 6 orders of magnitude (to make 4 billion years look like about 4 thousand), which would have changed the halo radius by 20% or so.
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