quot;When my sister is sad, my friend is upset and confused,

Dear piealpha,
This is a really tricky point. The -ing form of a verb can play three different roles
(a) participle modifying a noun (analogous to an adjective)
(b) participle as part of the progressive tense construction (functions as a verb)
(c) a gerund, which plays a noun-role in a sentence.
See this post for more on this distinction:
https://magoosh.com/gmat/2012/the-ing-form-of-a-verb/

Here, the word "questioning" is the object of the prepositional phrase "by", so it must be a gerund, not a participle. While it is non-standard ---- I have never seen it in official GMAT material ---- technically it is not grammatically incorrect for a gerund to be in parallel with a noun. It's not right enough to appear on the GMAT, but not entirely wrong either.

"... derailed by P and Q ...." That would make perfect sense if P & Q were both nouns or both gerunds --- either of those could appear on the GMAT. If one is a noun and one is a gerund --- well, that's somewhat non-standard, but not fully wrong, good enough to pass in somewhat informal conversation or writing, but not up to snuff for the GMAT SC.

Does all this make sense?
Mike

Thanks again to clarify. I got your point.