Has a sentencing manipulation defense ever held water?
It seems as if this form of injustice is considered possible but being ignored by the courts.
Many times.
See pages 4 and onward here.
The 11th circuit (which includes FL) recognizes sentencing manipulation but not sentencing entrapment.
The one example from the 11th circuit in that document (US v. Ciszkowski, 492 F.3d 1264) was an unsuccessful claim of sentence manipulation, but it shows the analysis that goes into deciding these types of claims. They say:
While our Circuit does not recognize sentencing entrapment as a viable defense, we do recognize the outrageous government conduct defense, and we have considered sentencing manipulation as a viable defense.
...
Ciszkowski, however, has not met his burden of establishing that the government's conduct is sufficiently reprehensible to constitute sentencing factor manipulation. Government-created reverse sting operations are recognized and useful methods of law enforcement investigation. Sanchez, 138 F.3d at 1413. The fact that law enforcement may provide drugs or guns essential to a willing and predisposed offender does not necessarily constitute misconduct.
We have previously declined to find that the government engaged in prohibited sentencing factor manipulation in other similar contexts.