Your mother's condition sounds like mine, and I have decided not to go to the stronger medication or ablation to resolve my AFib. I too have an enlarged left atrial, due to a leaky mitral valve that was repaired with open heart surgery about 15 months ago.
My cardiologist and his supporting electro physiologist (EP) both have told me that curing the AFib will not increase my life expectancy, nor will living with AFib reduce my life expectancy. This is an evolving prognosis and has changed in the 10 years I've suffered from AFig.
I have had good past results with electrocardioversion and medication (Propafenone). But the condition of the left atrial enlargement has prevented any further cardioversions using those "low level" measures. My surgery has stopped the enlargement of the left atrial, but too late to help me stay in sinus rhythm.
I too take 100 mg of Metoprolol to control HR, keep resting rate under 100, maybe as low as 70s when sleeping.