[thaumasiotes]

> In the simple past, the event is complete, whereas in the present perfect, the event is not, either because the event is ongoing, or because it maintains a specific relevance that means we are not looking from ‘outside’ the event as a completed whole.

This is an unusual viewpoint. The perfect aspect is so called for the fact that it indicates completion -- that is the literal meaning of "perfect". The fact that an action completed in the past may continue to be relevant in the present is not generally felt to indicate that such an action wasn't really completed.

> American English [...] does not normally require you to use the present perfect except for emphasis.

This just isn't true. The form in question is required to mark experiential aspect and it's also the only way of describing an action begun in the past that continues through the present ("I've been calling for 15 minutes").

Perfect forms are also required, as you note, when the speaker wishes to focus on a present situation related to an action taken in the past, as opposed to focusing on the past event itself. This is not rare.

And from the other direction, I'm not even aware that perfect forms can be used to provide emphasis.

[d1sxeyes]

> The perfect aspect is so called for the fact that it indicates completion -- that is the literal meaning of "perfect".

Ideally this would be true (and it is generally true in languages which are more concrete about aspect), but that’s not how it’s used in English tense names.

For example, ‘I’ve been playing tennis since lunchtime’ is not complete, but this is a form of the present perfect. Note that there’s also no indication of whether you intend to complete the action or not, and no distinction between actions which will or can be completed and those which won’t or can’t - apart from the latter sounding ungrammatical (such as the construction ‘I’ve been knowing’ - which is common in certain dialects but violates prescriptivist grammar rules).

> This just isn't true. The form in question is required to mark experiential aspect

I think actually you’re mistaken here. In British English, it is normal to say ‘have you seen the new James Bond film?’, while in American English ‘did you see the new James Bond film?’ Is also normal (although both can be used). In most British dialects, the simple past here sounds jarring and ungrammatical.

> it's also the only way of describing an action begun in the past that continues through the present ("I've been calling for 15 minutes").

This was a clumsy statement on my part. By ‘except for emphasis’, I intended to handwave over cases where something other than the action itself is being emphasised by the speaker (ie: duration, as in your example, persistent evidence of recency, such as ‘it’s been raining [and the ground is still wet]’, or some other dialect or ‘non-standard’ use. For example, often eyewitness statements, police reports, etc. unconsciously slip into the present perfect in cases for which there’s no standard grammatical explanation).