Is it possible, David, that "from where" works there because, as you said, the place being referred to is likely to be not just "the knoll" but "the knoll behind the mission"? Then, the referent is not just a noun (in which case the relative pronoun would be "which") but a locative phrase. Do you think this works?:
- They returned to the cathedral in the suburbs, from where they could observe the whole city.
I definitely agree about the locative meaning of the phrase, Gustavo. In these special relative clauses, "from where" means "from there"; the nonrestrictive relative clause can actually be rewritten as a separate sentence with "from there" replacing "from where":
- They returned to the cathedral in the suburbs. From there they could observe the whole city.
- She often climbed to the knoll behind the mission. From there she could look down on roofs and people.
While there is nothing preventing "which" from referring to "knoll" rather than "mission"—the noun phrase "the mission," whose head noun is "mission," is syntactically embedded within the higher noun phrase "the knoll behind the mission," whose head noun is "knoll"; thus, the relative pronoun "which" can take either head noun as its antecedent—I like your observation that, in phrases like "climbed to . . ." and "returned to . . .," the noun phrase complementing "to" (or forming its object) is a locative phrase, to which "where" may felicitously refer.
https://thegrammarexchange.inf...rom-which-from-where
This is a relevant thread, but I can't tell if Rachel provided a correct analysis.
Interesting thread. We haven't seen Mengxin in a while; I do miss the challenging questions that he would sometimes ask—but now we have you and deepcosmos (along with, lest I forget, the ever-present Navi). 
I sense that Rachel was a bit hesitant in her answer. I can't recall her ever referring, in any other thread, to how a professor of hers would have parsed something. Although I agree that Mengxin's textbook's exercise is grammatical with "from where" ("Alice stood in front of the window, from where she could watch her classmates playing football"), I do not think that it is right to parse "where she could watch her classmates playing football" as a noun clause (fused relative clause, free relative clase, whatever term you like to use for them). If that were the correct analysis, and the noun clause were the object of the preposition "from," then the sentences below should work; but they don't.
- *Alice stood in front of the window, from there.
- *Alice stood in front of the window, from it.
The "where"-clause needs to be parsed as a normal nonrestrictive relative clause, one which simply contains the preposition "from." Notice that "from" can even be stranded in the clause. This is perhaps not very elegant, but it is grammatically possible; and it shows that the preposition "from" is part of the relative clause, as it could not be if the "where"-clause were functioning as the object of the preposition.
- Alice stood there, where she could watch them from.
I'm still working on identifying the conditions under which "from where"-relative clauses (and perhaps other "[preposition]-where"-relative clauses) work, and I haven't been able to decide whether such clauses must be nonrestrictive. The problem is that I hear and read such clauses so rarely, and they come so unnaturally to me as a native speaker, that I really have to bend my mind over each case, to decide whether it is something I could say if I were inclined to say it, and whether rules may be extracted from that to describe the overall, extremely rare phenomenon.
