英文文法與句子結構解析(in which) ?

Millie has also become---s+v
unicef's youngest GA,---adj phr.
"a role"=limited co-ordinate phr.
 in which she speaks-----orally with the rel. pron as the object of a prep.to qualify Goodwill Ambassador, adj cl of reason, manner, condition
out on behalf of children----prep. phr
who need help-----adj cl of manner, condition;
in which=rel pron as obj to Goodwill Ambassador; hence the co-ordinate phr "a role" can be omitted.
The simplified original sentence eg:-
She speaks for needy children in UNICEF.(which is a limited role).
=limited in no. one role only (for her)
=one co-ordinate cl;
---If limited in no two roles (for her)
=2 co-ordinate clauses.
---all the rest are relative cl.as co-ordinated limited in number one or above in no.
---the relative clause(which)shows what thing is meant for HER to do. And to add more information to a sentence,ie in the middle of the main sentence, as this sentence is writing.
Also eg:-A surprised role came to Millie, which asked HER about Goodwill Ambassdor in UNISEF.(an adj cl).-------s-v-o-as co-ordinated limited in number one in no.only.

雖然我不是專家，但我嘗試以我所知解析。

先說 a role ，此處作為「UNICEF's youngest Goodwill Ambassador」的同位語（也就是意義與「UNICEF's...Ambassador」相同）。

再說到 in which ，這是一個關係代名詞（也就是具有連接詞+代名詞功能的詞類，簡稱關代）的更進階用法。

此處 which 即是該關代，代表的是前方的名詞（簡稱先行詞）， "a role"。後方再接句子（空出所要修飾的名詞的位置），修飾先行詞。

而如果後方接的句子留下的空缺，前方有一個介系詞(in/at/on/...)，則可以依照寫作者的決定，看看要不要移到關代前方。（可能為了易讀性、通順不通順考量。通常移了比較正式，不移比較口語。）

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所以我們來假設一下這個句子的演變
Millie has also become UNICEF's youngest Goodwill Ambassador, and she in this role speaks out on behalf of children who need help.
=> Millie has also become UNICEF's youngest Goodwill Ambassador, a role which she in [] speaks out on behalf of children who need help.
=> Millie has also become UNICEF's youngest Goodwill Ambassador, a role in which she [] speaks out on behalf of children who need help.

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不過呢，關代有分「限定用法（用來限縮範圍）」與「非限定用法（用來補充說明）」。非限定用法通常在前方加逗號，我個人認為有些時候也可以把它看成 同位語 + 疑問詞子句（代替名詞） 的用法。

題主您所提的句子是使用限定用法的，用來限縮 a role 的範圍，變成「不只是一個角色，而是為有需要的孩童大聲疾呼的角色」。

我認為這一句其實也可以寫得更簡單一些，即：
Millie has also become UNICEF's youngest Goodwill Ambassador, in which she speaks out on behalf of children who need help.