How often does (1) create a misunderstanding among native speakers?
This comes up in my life quite often. For example, whenever I'm on a train with someone and they say "Выходим через остановку", I feel the urge to clear things up with:
- Ты имеешь в виду «на следующей»?
- Нет, через остановку.
That usually does it. But if I'm given the directions "Сядь на «Курской» и выйди через три остановки", I have to make really sure what they meant if I don't want to end up in the wrong station. Because it could mean either:
- Проедь три остановки и выходи на четвертой, or
- Проедь три перегона между остановками и выходи.
Those will get me to different stations.
This meaning of через is quite common when speaking about recurring events or station stops and it's distinct from the meaning of "через <time span>".
It's very similar to one of the spacial meanings of через which is over: Перепрыгни через ручей = Jump over the creek. Imagine you jumped over three narrow creeks in one long jump. In practice that would mean you jumped four distances between creeks:
If there was a fourth creek, you'd be right in the middle of it. That's the kind of thinking that makes "через три остановки" mean "skip over three stops and get off at the fourth".
So when your teacher said, "На следующей неделе не будет, будет через неделю", what they meant was:
- We are in week 1.
- Let's skip the whole of next week (week 2).
- This gets us into week 3.
"Через неделю" does not create any ambiguity here because, as your teacher said previously, it's not next week, so it must be the week after.
Why is that so (2)? like some kind of sneaky grammar rule that everybody learns in school, but then forgets about.
Often, when we see or hear or read something, we give it just enough thought to find one interpretation and then take it as the only one fathomable.
For example, when we look at the spinning ballerina, we can only see her spinning in one direction, although with some effort we can make our brain see her spinning in the other direction.
Or, when you read or hear "Time flies!" your first thought would probably be "Yes, it does" and not "What on Earth do flies have to do with time?"
Similarly, when you give people the phrase "через неделю" they think of the most common interpretation, "in one week". It's only when you show them the context, they go, "Oh, it's the other через!" Yes, через has at least these two meanings, "in" and "skipping".
I'm not sure if this psychological phenomenon has a name. Let's call it "Slepov's first interpretation law". Or just laziness.
In the given context, can "неделю назад" mean "14 days ago"?
No. I might be falling victim to my own law, but I can't think of a context where it would mean that.