Quom for the preposition cum is considered erroneous, I think, though it occurs in some inscriptions and MSs by confusion with the conjunction quom/cum. This verb is most of the time intransitive, and is so here. Litterae in this sense usually translates to the singular "letter" and, vice versa, "letter" in this sense normally translates to the plural in Latin. That's because a letter (brought by a messenger) is made up of many letters (of the alphabet). Pretty much always "from", really, except when used before the agent of a passive verb, where it translates to "by" (as in Caesar a coniuratis interfectus est, "Caesar was killed by conspirators"). "Of" isn't a usual translation of a(b), though I guess there may be some exceptions (I know there are in medieval Latin). The former. The ablative of Iulia would be Iulia (with a long a). See above regarding "letters" and "Julia". "Forth" seems unnecessary. The rest is correct.