A Japanese friend of mine told me that ② is not wrong but ① is better than ②. Could any expert tell me why の is better than が in this sentence (and the interchangeability of が合わせる and の合わせる)?
the interchangeability of が合わせる and の合わせる
You don't seem to be parsing the sentence correctly, I'm afraid.
この事故による死傷者は、女性3人、男性4人の（合わせて）7人ということになる。
The 合わせて is an adverbial phrase, meaning "in total". The の continues to 7人, not to 合わせて.
女性3人、男性4人の7人
The の is appositive. 「女性3人、男性4人」 is in apposition to 「7人」.
A few examples of appositive の:
「弟の次郎」"my brother Jiro"
「羊のショーン」 "Shaun the Sheep"
「リンゴ1箱とミカン1箱の計2箱」"a box of apples and a box of oranges, two boxes in total"
For more on this, you may want to read:
I think ② is wrong. To clear the problem up, I change the details. 死傷者は、女性3人と男性4人の、合わせて7人だ。 So you can realize 合わせて is just a kind of adverb. 死傷者は、女性3人と男性4人の、7人だ。 Reading this sentence, you know that の is a part of adjective like の in 黒の服, 昔の話. 死傷者は、女性3人と男性4人が、7人だ。 I think this can't make sense though I can imagine what it means.
