Unfortunately, whenever Hyper-V is enabled on the host, the host OS will itself be running inside a Hyper-V partition, so everything running on the host OS is subject to Hyper-V's limitations. We have no way to interpose ourselves at a lower level than the host OS's integrated hypervisor. A host with Hyper-V enabled will therefore only be able to run applications for which Hyper-V can provide a suitable virtual environment.
So, as far as I know, until Microsoft enhances Hyper-V to provide the virtual CPU features/fixes required for it to safely run nested virtual machines, it will not be possible to run Workstation on a host with Hyper-V enabled.
I hope this helps explain the situation...
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Darius