I'm actually am having a little trouble grasping your meaning with these questions. I agree that someone giving advice on (1) should have applied (1). Otherwise, they don't have a justified claim to the knowledge in (1). But this is the case whether or not they demonstrate (2), which is why I'm confused by the wording of your question.

What I don't get is why it is relevant if the advice-giver failed to realize that they should have applied (1) in some particular case, even if they ought to have known that they should have applied (1).

To give a gruesome example, a professional hitman might be able to give very good advice on how to kill someone you've decided to kill, even if his advice on when to decide to kill someone is spectacularly bad.

Similarly, your evaluation of Alicorn's advice on how to like someone you've decided to like should be independent of your belief that she's very bad at deciding when to like someone.

From the very first sentence the underlying premise supporting that 'how' is that the author can, in fact, do the thing in question. It isn't presented as something known in theory, were that the case external evidence would be required, not implicit reference to personal experience.

I don't see what you're saying here beyond what I myself said here, when I wrote,

I agree that someone giving advice on (1) should have applied (1). Otherwise, they don't have a justified claim to the knowledge in (1).

To be a little more explicit, I meant "applied (1) successfully". As I go on to say, this does not contradict the distinction between (1) and (2), because:

What I don't get is why it is relevant if the advice-giver failed to realize that they should have applied (1) in some particular case, even if they ought to have known that they should have applied (1).

To give a gruesome example, a professional hitman might be able to give very good advice on how to kill someone you've decided to kill, even if his advice on when to decide to kill someone is spectacularly bad.

Similarly, your evaluation of Alicorn's advice on how to like someone you've decided to like should be independent of your belief that she's very bad at deciding when to like someone.

(I'm quoting myself at length here because someone downvoted me earlier for giving just a link to another comment when I thought that the other comment said all I would want to say.)

You go on to say,

This being the case either a demonstrated strength in the area or a description of specific improvement in a weakness is required to give support to the 'how to guide' in question.

I think that this is a form of asking for impossible evidence. Of course, the evidence you request is not really impossible. Alicorn could have given all sorts of identifying details of the people she forced herself to like, and she could have described at length the circumstances under which she did so.

However, it's not reasonable to have expected her to do this in the OP. The social sanction against doing that kind of thing is too great, and with reason. It would not have helped the reception of her article to drag forth all of her grievances and peeves against someone, just to describe how she overcame all these issues and learned to like the person. To expect this of her is to have an unrealistic picture of human interaction.

Therefore, Alicorn's lack of "description of specific improvement" is not Bayesian evidence against her ability to do what she advises, nor against the possibility that she has applied her advice with success. We just have to evaluate the plausibility of her hypothesis by other means, such as consistency with our prior knowledge and our own experimental tests.

With extra rejection given to the condescending tone.

My blunt tone is intended to be a sign of respect to Silas. One of the things that I admire about him is that, when he disagrees with someone, he says so plainly, often without expressing contempt (though not often enough). He does not obscure his position by softening it to save feelings. I extend to him the same courtesy.