(May I call you Rachel?)
I appreciate your help and comments.
I was just thinking about the difference in role between the "that" that introduces a complement clause and the "that" that introduces a relative clause.
I think (if I'm following you right) that what you're saying is that for native speakers who judge (i) as being ambiguous, the difference comes down to (ii).
(i) Who did Philby know that Angleton suspected?
(ii) a. asking about the person(s) who Philby knew and
who Angleton suspected
b. asking about the person(s) who Philby knew that Angleton suspected
Dulles was a/the person that Angleton suspected, Philby knew.
I have long wondered why something moves across the verb "know" from within its object noun clause with far greater ease when the verb appears in a relative clause than when it occurs in an interrogative one. Topicalization may be a clue.
Probably, even English-people who deny the readings of (2b) will think about it again if (i) appears in something like this:
A: Philby knew that Angleton suspected him.
B: Him who? Who did Philby know (that) Angleton suspected?
I'd like to seek this possibility somewhere.(I wish my mind could fly like a bird, sadly my brain can't think as fast as I want it to do.)
Thank you again,