Sin, Sun, stain, skin, spin, spun, span, scan, shun...
sanitation
Lighthearted is a 12 letter word. Lithographed is another 12 letter word that starts with L and ends with D.
Arithmetic
The only English word I could find that meets the criteria is candleholder, but I am not sure it is a hospital helper.
Suffragettes.
handsomenessharmlessnesshelplessnesshomelessnesshopelessnesshorribleness
denomination
lsjfl
* contribution * colonization * cancellation * civilization * constitution * commendation * compensation * condensation * congregation * confirmation * conversation * conservation * constipation * continuation * coordination
exaggerative, exhilarative exaggerative, exhilarative
Exterminable
chauvinisticcatastrophic
2 Main Words are Child-prodigy and Cross-country.