I don't have the full answer to your question but I do have an answer to this:
Furthermore, does the answer depend on whether we directly undergo the experience firsthand or merely hear about it through others' accounts (i.e., secondhand or thirdhand)?
I think it HAS to matter if you had it yourself or not. The world is full of people claiming mystical experiences, and a huge portion of them are mutually exclusive. Joseph Smith's mystical experiences are mutually exclusive with, say, L Ron Hubbard's.
When it comes to religiously specific mystical experiences, at the very least MOST of them have to be "incorrect" in some way - "incorrect" takes a wide range of meanings here, from "the person is lying about or misremembering their experience" to a more benign "they are misinterpreting their experience" to "it was just a dream/hallucination and nothing more".
And when you realise that they have to be mostly incorrect, in one of those types of ways, you then realise that you can't take for granted ANY of them. You cannot make any significant theological conclusions based on the mystical experiences other people report, including people you know and trust.
You should seek your own, AND you should be willing to consider interpretations of that experience that are less obvious (including questioning it entirely).