Now I know (1) may not be ambiguous to some native speakers, and the "that" clause is taken to be a relative clause (but not a complement of the main verb).
(1)Who did Philby know that Angleton suspected?
Then we can explore the possibility that we can replace "that" with a wh-word.
My question is,
Do you accept all of the three below?
(2) a. Who did Philby know who Angleton suspected?
b. Who did Philby know whom Angleton suspected?
c. Who did Phlby know φ Angleton suspected?
By φ (phi), I mean the place φ occupies is nothing you can see or hear. Or, the relative pronoun is omitted or deleted.
Normally, where "whom" works, "who" and "φ" also work, but here the relative clause modifies the interrogative "who." In such cases, we can't use "φ". So I've learned.
What's bothering me is, especially whether the general rule applies to this particular case.
Thank you in advance