Yes, all the 3rd person pronouns start with a vowel and that is how the rule is applied. As I found, "Pronouns that start with vowels may be proceeded by the letter "н" when used with some prepositions."
It actually can be. There is a verb "to have" in Russian. But, it's a culture thing. An example for, let's say, English-speaking people in the USA: saying "motion picture" instead of "movie" or "film" used to be a thing. Now, if used by a person at all, it's used very seldom. "Movie" and "Film" took over as the two words that the vast majority of people ever say; our culture changed. The whole "u ... est'" thing must have become so popular that it became the norm.
tl;dr You can say the sentence like that and you would be understood, it would just sound weird to most people.
How do you differenciate У них есть between a question and a statement? I see it come up as both 'they have' and 'do they have'. In text it is a bit easier with a question mark, but you dont get that in conversation, nor does scanning an entire sentence for the last character first make much sense.
In text it is a bit easier with a question mark, but you dont get that in conversation
In conversation you use the tone of your voice to make it a question.
nor does scanning an entire sentence for the last character first make much sense.
It makes sense for millions of Russians and native speakers of other languages that don't use inversion for questions. It's really not that hard.
There's no secret or a hidden trick here. A question mark in writing/typing and intonation in speech. That's it.