We are trying a scenario where unlocking a page in AEM 6.3 is failing. PFB the steps followed in a vanilla AEM instance.
Please let me know if this is the intended behavior in AEM 6.3 and what will be an alternate solution for the same.
From my experience in my project, either admin OR user with admin rights will be able to unlock the page. A mere impersonation of the person who locked the page does not give you all the access to unlock the page.
Also, did you get a chance looking into some of the options that were discussed in this thread. Unlocking Pages - Alternative to "admin" Account?