Salam,
According to Hans Wehr and Lane's Lexicon:
Umm (Alif Meem) is defined as:
1. to go, betake oneself, aim at or pursue (something or toward some place)
2. to lead the way
3. mother, source or origin, basis or foundation
ummatan (alif, meem with the added ta murbuta) the definitions listed are:
Lane's Lexicon
1. A way, course, mode, or manner, of acting,
or conduct, or the like
2. one course in which people follow in the way of religion
3. any people called
after a prophet e.g. followers of the prophet
4. A nation; a people; a race; a
tribe, distinct body, or family
I would say that the first and second definitions (with the ta murbuta added) combined correctly defines the word at this point (a way, course, mode, or manner of acting in the way of religion)
you must also keep in mind that kana is defined by Lane's lexicon as "strong in make" so change the definition for kana from "was" to "strong in make" and you have the following new translation:
inna - That
ibrahima - Abraham
kana - strong in make
ummatan - a way, course, manner of acting in which people follow in the way of religion
qanitan - obeying humbly
lillahi - to God
hanifan - true believer (I changed this according to Hans Wehr pg 245)
walam yaku - and not he
mina - from/of
al-musyrikina - the idolaters
Salam,
Arnab