Laudable to say the least...
Thank you. You are very kind.
And does this stem principally from extensive practice in the field (to the point that, now, English syntax seems just as natural to you as that of Spanish)?
Actually, grammar has always been my favorite subject in all the languages I have studied, and English is no exception. Understanding syntax is, as you know, essential for translation. I love syntactic analysis, but I'm only versed in traditional grammar. David, our moderator, is much more qualified than I am, and masters both traditional and generative grammar, apart from being a highly educated native speaker.
I also have one more question regarding my original post. Do you know where this construction/structure is treated in either The Cambridge Grammar of the English Language (Huddleston/Pullum) or A Comprehensive Grammar of the English Language (Quirk/Greenbaum)?
I can direct you to A Student's Introduction to English Grammar by Huddleston & Pullum, which on pages 185/186 says:
R element (relativized element) within an embedded clause
It is possible for the R element to be located within a content clause embedded inside the relative clause:
 i a. a key [which he says she found] b. a key [(that) he says she found]
ii a. some boys [who he says saw her] b. some boys [(that) he says saw her]
In [i] R is object of found, and the found clause is a content clause functioning as complement of says: "He says she found R". We understand that he says she found some key, and that's the key the whole NP refers to.
In [ii] R is subject of the embedded saw clause: "He says R saw her". Note that the that is omissible in [iib] : this differs from [6i] above (some friends that saw her) in that the R element is subject not of the relative clause itself but of the content clause embedded within it.