What symptoms would someone with very low ejection fracture number (12%) show? Possible they can seem healthy & do normal activities until heart fails?

Many. People with a 12% ejection fraction (ef) are considered to be in severe heart failure. The symptoms of heart failure are shortness of breath, fatigue, swelling of the lower extremities, fluid build up in the abdomen (ascitis), chest discomfort, occasionally cough, weight gain due to fluid retention, decreased urinary output, intolerance of physical activity etc.
Differences. The patient may be anemic for which muga scan will not be reliable. Sleep apnea can surely change ef. After it is corrected ef god willing will improve. Ie any time heart does not get oxygen rich blood, ie broken nose, sleep apnea, asthma, copd, pneumonia, blocked arteries, htn, fast heart rate, anemia (blood carries oxygen) . When you correct this with meds and this aid the heart.
Weakness, fatigue. CHF at that low level, one can see shortness of breath, weakness, and swelling in the legs, however, if the heart failure is compensated and the blood pressure is regulated, they may not have any symptoms, they just need to be evaluated for an implantable defibrillator as the risk for sudden cardiac death is higher. This would only be done if the heart failure did not improve with meds.