A few extra comments on The City of God

(ii)
You really need to pay careful attention to Sidwell's notes. If it helps, look at the notes first, find what they refer to in the text, and somehow mark those words or phrases (I'd put a little asterisk before the first word), just so you don't miss a note.

line 2: hoc: despite the alius, A. uses hoc since he is thinking of the bonum that follows. hoc .... bonum forms a noun phrase, with the adjectives that modify the noun in between the demonstrative and the noun. Of course, in English we have to make the adjectives predicative: hoc bonum est pariter.... et coaeternum.
3: nec ... non: another double negative.
4: propter hoc: hoc refers to the quia clause that follows; I would translate the quia by something like "in that", "inasmuch as".
5: uero: "moreover"
    sola: "only"
8: utique: one word, look it up;
9: in quo ... alterum: Sidwell's capitalization and punctuation could be misleading. If you read the Latin as he suggests, it should be: "In the respect in which it is spoken of in relation to itself and not to another,...." I would suggest translating in quo as "In as much as..." or "In that". The in might be redundant, or A. might be thinking of quo as a proleptic (that is, coming before) relative pronoun, referring to the idea of the clause ad semet ipsum dicitur.
10: hoc est: word order is est hoc (as you would expect in Latin, with the verb coming at the end of its clause) and quod habet is a relative clause.
11: dicitur uiuus: "it is said to be alive," "it is called 'living'".

(iii)
1: itaque: delayed, as usual: put it first.
    natura: sc. "Trinitatis".
    sit: in Cassell's, s.v. (1)b, "with subj.".... (The subj. is there because it is a type of relative clause of characteristic).
    uel: not "or"; in Cassell's, s.v. "Adv. (1)."
5: Hinc est quod: or "Hence there is the fact that...."
9: secundum: preposition
    dicuntur illa simplicia: read as illa dicuntur simplicia.
    uereque: uērē + que
10: quod: relative conjunction
11: vel ... vel ... vel: conj., in Cassell's s.v. (2)
     sunt: delayed, as usual in Latin, but you'll want to move it to the start of its clause