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