Is it possible to give an Employee Center role access to POs whose Employee associated with is not the user and not his or her subordinates?

No. This is a limitation of the employee center role. I found this when attempting to configure suiteapprovals for such a use case, and found that even though suiteapprovals can be configured in this way the users would never be able to open the POs. If you were to use SuiteApprovals you could turn on Email Approval and approve that way (where they can view the attachment), but I'd recommend against this as it's an incomplete solution / poor workaround.

Got it. Thank you for the confirmation.