Is the expiation for breaking my fast—feeding a poor person—obligatory upon me for the days I missed and later made up, or does it lapse due to inability, and would it become obligatory upon me again if I become able to perform it?

If breaking the fast is due to an excuse, there is no sin. Otherwise, it is a great sin that necessitates repentance and making up the missed days (qada'). Since you have made up what was required of you, praise be to Allah. The majority of scholars obligate feeding a poor person for each day of delaying the makeup of Ramadan without a valid excuse until the next Ramadan begins, except for someone ignorant of the prohibition. This expiation (kaffara) does not fall away due to incapacity; rather, it remains an obligation until one is able to pay it.