Not fasting on Yom Kippur

Properly eating on Yom Kippur for medical reasons is a mitzva, not a sin. It needs the guidance of a Rav, but if done properly its as important as fasting would have been.

If you have to eat/break the fast for medical reasons, it actually becomes a mitzvah to do so. Now, obviously it’s not something that’s taken lightly- that’s why most people who are unable to fast are told to do “shiurim” and even that is only allowed under the most pressing of circumstances, but because the halacha is that one who is ill enough must eat, doing so is actually following halacha. A person who goes in knowing they won’t be fasting from the get-go is not sinning (assuming they have consulted with a competent halachik authority), so there is no issue of “sin and then do teshuva”.

shlishi -He isn’t Mekayem Vchai Bohem because then he would be able to eat regularly. But he is Mekayem V’nsheshmartem Moed Es Nafshosaichem. And eating less than the Shiur is also Ossur because of Chatzi Shiur Ossur Min Hatorah!

shlishi- ditto to sam

Health- depends if chatzi shiur is an issur chatzi shiur of what the torah assured or an issur unto itself. if like tzad 2 it wouldnt be altz achila.