How to render the content in which the user is mentioned in?
If you think about writing this as a pure SQL query, what you are trying to do is have multiple WHERE conditions that are separated by OR operators. 

Using OR operators with Conextual Filters is not possible out-of-the-box with Views. All Contextual Filters are added as AND filters.

There appears to be a module that adds this option, although I have not tested it. It doesn't have an official release, but appears to be in use by over 2000 sites, so might be worth a try: https://www.drupal.org/project/views_contextual_filters_or

Ultimately, however, this could be solved with a different data architecture. Is there a reason you have multiple different fields for referencing the user? Could these fields be combined into one base field, with instances on various content types? If so, then you can do it with a single Contextual Filter.