Understanding "を手伝う"

In the sentence "Could you help me with my homework" 宿題を手伝ってくれませんか, I'm trying to understand why を is placed here.

を marks the object which is being acted upon. The action is help, and I am being helped, therefore shouldn't the を be placed after 私 (although there is no "me" in the sentence above)?

宿題 is an action noun(動作名詞), and it performs as 宿題をするの/こと there. In this case, を indicates the work the subject helps with. Please refer to the example sentences in a dictionary below.

てつだう【手伝う】
I〔手助けする〕help ((a person with))；help ((a person (to) do)) ；assist
彼が私のコピーを手伝ってくれた
He helped me with the photocopying.

その老人がバスから降りるのを手伝ってあげた
I helped the old man (get) off the bus.

彼がオーバーを着るのを手伝った
I helped him (to) put his overcoat on.

ちょっと手伝ってちょうだい
Help me for a minute.／Give me a hand.

As you can see in the first example sentence, コピーを手伝う means コピーするのを手伝う, as same as バスから降りるのを手伝う and オーバーを着るのを手伝う in the second and third examples. In other words, these expressions can be rephrased as 老人のバスからの降車を手伝う and 彼のオーバーの着衣を手伝う by using active nouns. (As you might already realized, 宿題 is actually 私の宿題, so 宿題を手伝う means 私が宿題するのを手伝う in your sentence.)

Of course, the one who is helped also can be indicated by を like the final example, as you interpreted, but it's mostly when the work is not mentioned.

Some verbs can be used in similar expressions.
e.g.
彼の留学を許す（＝彼が留学するのを許す）
彼女の研究を助ける（＝彼女が研究するのを助ける）

It works the same way as "teach" 教える. Like if you say 英語を教えてくれました. It's "me" that's being taught in the same way that "me" is being helped.

Well, I don't think it's similar to 教える in this case. 英語 is the direct object and "me/the one who was taught" is the indirect object of "to teach" as same as 送る or 話す.

私に英語を教える
teach me English/teach English to me

彼女に手紙を送る
send her a letter/send a letter to her

彼に噂を話す
tell him a rumor/tell a rumor to him

に is not used to indicate "target/indirect object" of 手伝う. When the object is "person/people", を is used, as I already wrote.

○母を手伝う
○母の料理を手伝う
×母に料理を手伝う
(Notice that 料理 is also an action noun.)

The point is that the object of 手伝う is not necessarily "person/people" in Japanese, unlike "to help" in English.