How can one make up for fasting in previous years, and is expiation or charity sufficient instead of fasting, given the inability to determine the number of days or the difficulty in making them up?

You must make up the days you missed from Ramadan. You should fast as many days as you believe will absolve you or as you are certain will suffice. You are not required to make up the days consecutively; you may do so separately, as is convenient for you. If you do not make up the days before the next Ramadan begins without a valid excuse, you must also pay an expiation for delaying the make-up fast. However, if the hardship is unbearable, even for fasting a single day, and the inability is continuous, this means you are also unable to fast Ramadan. In such a case, you may break your fast and feed one poor person for each day, and you are not required to fast Ramadan nor to make up its missed days.