Understanding 「同居している25歳の女性に暴行を加え殺害したとして」

Question

I'm having trouble understanding the bolded parts from a sentence from the first episode of Death Note:

本日正午過ぎ東京都渋谷(しぶや)区で⸺同居している25歳の女性に暴行を加え殺害したとして

Questions:

1. I'm assuming the single verb "同居している" makes up an adjectival relative clause, but what is it modifying? Is it "25", "歳", or "女性"?
2. I know 暴行を is a direct object, but is it modifying 加え or 殺害した or both?
3. For that matter, what is 加え殺害した? I know that 加え is the stem of 加える, and that 殺害 is being combined with する to form a verb. Is 加え a noun that's being combined with する also (alongside 殺害)? Or is the whole of 加え殺害した a single verb (if so, what does it mean)?

Answer 1

1. 同居している modifies the noun phrase 25歳の女性 ("a 25-year-old woman") as a whole. You need to use your common sense; a human cannot live with "twenty-five" or "years old" (see Are Japanese modifiers "greedy", "anti-greedy", or do they mean whatever people choose them to mean?).
2. 暴行を is the object of only 加え. 暴行を加える is a common set phrase, but 暴行を殺害する ("to murder an act of violence") does not make sense.
3. 加え here is used as a 中止法 of a verb. The sentence is roughly the same as 女性に加えて殺害したとして. See also: Removal of て in Japanese novels