grammaticality - "A wants B to guess that..." or just "A wants B to guess who/what/where/how..."?

I am testing whether "A wants B to guess..." can be followed by both declarative and interrogative clauses. I am not a native speaker but "A wants B to guess that.." does not sound acceptable to me.

Do you think they are natural sentences in English?

[1a] Mary wants Sue to guess who John talked to.

(This should be perfectly natural, right?)

[1b] Mary wants Sue to guess that John talked to Prof. Smith.

(If this sounds natural, how do you interpret this sentence?)

As a contrast, compared with 1b, what do you think about

[1c] Mary wants to guess that John talked to Prof. Smith.

(Is 1b better than 1c?)

Thank you!