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, ...
"I'd want to" is short for "I would want to" and is common usage for most Americans. Perhaps your area doesn't use that particular contraction often and is why you think it is not common.
And yes, your example sentence
"If there is one thing that I'd want to say about my brother, it's that he's very stubborn."
is grammatically correct. Your sentence is saying
"If there is one thing that I would want to say about my brother, it's that he's very stubborn.
Best Answer
AmE speech typically mixes up lay and lie.
"I may just go lie down on the bed"; the action of a person is: lie down
"I will lay your clothes on the bed when they dry.": what you do to a thing
In colloquial American speech, lay down is used for lie down. It is marked as dialectal and probably uneducated speech. (Uneducated speech does not mean I am judging it here. I am not.)
Lying down on a bed can make you feel better if you are not feeling well.
Laying your clothes out on the bed on a chair is a useful way make your mornings easier.
Please note the verb forms:
I lie, I lay [yesterday], I have lain [gerund: lying]
I lay, I laid [yesterday], I have laid [gerund: laying]
There are many sites on the Internet with exercises for these verbs, which many people do mix up, even native speakers.