What about the days for which expiation was given for breaking the fast due to an excuse, and must they be fasted again after the excuse is gone?

The criterion for an illness that permits breaking the fast is a severe illness that intensifies fasting, or for which there is a fear of delayed recovery, or the occurrence of unusual hardship. If you experience hardship due to your illness or fear its intensification or delayed recovery, then you are permitted to break your fast, and no fidyah (expiatory feeding) is obligatory upon you, as long as the illness is expected to be cured.

Since you found treatment and were able to fast, you must make up the days of fasting you missed, including the days for which you paid fidyah; this is because feeding is not a substitute for fasting except in the case of permanent inability to fast due to an illness from which recovery is not expected.

If trustworthy doctors determine that the illness is chronic, then in this case you break your fast and feed, and making up the fast is not obligatory. If you subsequently recover from it, then you must make up the fast, according to the relied-upon opinion in the Shafi'i and Hanbali schools of thought.