To your first question:
I have been going on package holidays for years and I have never had any problems.
The Present Perfect Progressive has the meaning of "lately" or "recently." There is no indication from the second phrase that she stopped going on these trips long ago in the past. In fact, it sounds like she still may be taking trips, thus the answer.
The answer to your second question also lies in the second phrase. He is referring to the fact that he was never sick before from the foods he had already tried, not the food he is continuing to try, because obviously the most recent food made him ill.
I have been exploring the world...
The distinction is slight, but perhaps can best be explained as, there is no indication that the subject will stop the activity. The alternative
I have explored the world...
does not hold that implication. It could easily end with
I have explored the world for over 15 years, and it's time for me to settle down.
I think what the page meant by saying "when, where, who" is that the perfect form doesn't go well with expressions suggests a particular time. The example sentences given at the linked page seem to be all about it.
The default past tense is the simple past, as Michael Swan says in his Practical English Usage (§421.4 "In general, the simple past tense is the ‘normal’ one for talking about the past; we use it if we do not have a special reason for using one of the other tenses.")
The perfect forms are needed basically to add the sense of completion to non-finite verbs such as infinitives, participles, and modal verbs. Because such tool exists, it's also used to talk about past events, but it ends up saying 'up until now' ('up until sometime ago' when it's the past perfect). Because it's tenseless by itself, it makes the sense of time vague, thus it has the sense of duration at the same time it conveys the sense of completion.
The use of the perfect forms are more to do with meaning than tense. Grammar books usually says it's one of tense form, but actually it's not about tense. Linguists call it 'aspect'.
The perfect form goes well with expressions like already, recently, just, since, ever, never. But it doesn't go well with expressions of particular point of time, when, such as yesterday.
I've read something more relevant to the OP's question, in the same M. Swan's PEU:
§457.1 (...) we usually prefer a past tense when we identify the person, thing or circumstances responsible for a present situation (because we are thinking about the past cause, not the present result). Compare:
Look what John's given me! (thinking about the gift)
Who gave you that? (thinking about the past action of giving)
Also
PEU §456.5
We normally use the present perfect to announce news. But when we give more details, we usually change to a past tense.
There has been a plane crash near Bristol. Witnesses say that there was an explosion as the aircraft was taking off, ...
Best Answer
The perfect from "Who has watched it?" would be normal. You are not concerned with the time when it was watched, only the the whether they now have the experience of seeing the film.
That doesn't make the past tense wrong. If you had already established that you were talking about a particular time in the past, you might choose the past tense:
American speakers are more likely to use past simple instead of perfect tense. British speakers are more likely to use perfect.