Is there a word that describes the phenomenon, often seen on SE sites, where someone says they are asking a question "for a friend", but actually mean themselves?
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One expression which encapsulates the phenomenon to which you refer is "under the guise."
On call-in talk shows on the radio, for example, a caller might preface his or her comments to the psychologist or psychiatrist by saying,
You know what they say:
As an armchair psychologist, however, I commend the person who calls about his "friend's" addiction. Despite their pretense, at least they realize there's a problem in their own life and are seeking help, albeit indirectly.
As for an OP who asks a question supposedly for a friend, I see nothing particularly wrong with that, unless the OP's motive is to deceive or otherwise hurt another person. Sometimes we just find it hard to admit being ignorant, whether the reluctance stems from pride and/or fear of what others (including peers) might think us, or from some other relatively harmless but perfectly understandable motive.
Another expression which comes to mind is feigned ignorance. One interesting aspect of feigned ignorance is that it can proceed from an ironic perspective. Socrates had mastered the art of feigning ignorance in order to encourage his interlocutor to talk himself into a corner, so to speak. Once that happens, the person who feigned ignorance can then pounce on his prey and eviscerate him verbally, exposing the interlocutor's true ignorance.
It's usually used with more the sense of experiencing another person's circumstances, but you can also act vicariously. In the context of vicariously asking for personal advice, you present someone else's circumstances in lieu of your own. But that other person (who may not exist) doesn't actually get the advice - you do. From Merriam-Webster:
Consider the term projection
This concept is used extensively in psychology to describe the unconscious attribution of personal thoughts or traits on another, but it is applicable in everyday usage as well.