Does an employer have the right not to accept a doctors excuse for an ongoing medical condition?

I'd say yes, for sure. The note might not be specific enough.  It might be stale-dated.  Might be from a doctor who doesn't customarily treat your condition (like getting a dentist to give you a note for a mental condition).  In any event, your employer might not be obligated to give you time off, modify or hold your position due to a medical condition.  You'd need to provide addl facts to get a more specific answer.