Is the Mycelium HD wallet private key stored encrypted on the device?
Yes, Mycelium stores an encrypted copy of your key, protected by your PIN (if applicable) on the device. This includes any individual private key accounts you import.

In general, I would advise against storing crypto keys (or any private keys or confidential information, really) on a corporate device (laptops, phones, desktops, email accounts), or even a device others can access regularly.