Yes you do, unless there is some legitimate medical reason for your inability to attend, in which case you may be able to delay your appointment. OWCP has every right to scheduled medical exams for you at “reasonable” intervals.
Not necessarily.
Your condition must substantially impair a major life activity, but still allow you to provide useful and efficient service to your employer in the position held or in another available bona fide position within the agency for which you are qualified.