With the variations in spelling, is it possible Hitrovo may also be Khitrovo?
Yes, the Cyrillic letter x (pronounced like the ch in Scottish
loch and German
Bach) in Хитрово is sometimes transliterated h and sometimes kh.
Just like foreign names starting with an h (sound and letter not found in Russian), like Hendrikova used to always be transliterated with a г (Гендрикова, Gendrikova; Гессен, Hessen), but now (new names, not those with old, established spellings) are transliterated with a x (Хиллари Клинтон, Hillary Clinton).
Is she possibly related to Margarita Khitrovo?
The designer Olga Hitrovo, née Шепелев(ова)-Воронович / Schepeleff-Voronovich, was married to Margarita's brother Vladimir.
See this Russian genealogy site listing all the siblings:
http://ru.rodovid.org/wk/%D0%97%D0%B0%D0%BF%D0%B8%D1%81%D1%8C:582567Feodor Oom was Countess Olga's stepfather. His patronymic name was Feodorovich. Was he the son of Empress Marie's secretary Feodor Oom?
That's correct. Son of Fyodor Adolfovich Oom (1826 - 1898), from a Baltic German family, see this thread:
http://forum.alexanderpalace.org/index.php?topic=3539.msg75833#msg75833