If you are contracted as an Understudy for a Lead, and you do have the opportunity to move into the role due to Lead becoming ill, is it then acceptable to list the actual role on your resume?

Yes, you can put it on your resume, as it is also listed in playbills. If you are onstage as ensemble when not the understudy, then you can list it as Ensemble/u/s Name of character or Ensemble/Name u/s. If you are onstage only as the understudy, then you can list it as Name u/s, or vice versa, u/s Name. Your preference. I've seen it both ways.