Can I apply for a "potential" deferred admission
You can always ask, but as a committee member I would probably vote against such an application unless the candidate is exceptionally good. When departments evaluate openings for PhD programs, they have needs they seek to fill: For teaching assistant positions, to keep the strength of a program, to spend money they might have, to have enough students in graduate classes, etc. All of these needs are timely: They need to be satisfied in the upcoming academic year. So the department is looking for people for next year, not the year after: There will be another crop of applicants to consider for the year after. So unless there are really compelling reasons to admit someone now knowing that they may or may not show up only a year later, departments will generally want to admit people they know will actually come.