You’ve highlighted a couple of large and well-established issues! I’ll point you to some resources here.
Here’s a (long) discussion of this topic and a place you can vote for a change:
It’s not actually a duplicate; it is the same (sub)task just being displayed twice in two different places. That’s why if you change one, it changes the other, because it’s literally the same object.
Whether to use subtasks or not is pretty much of a religious argument here - some swear by them, many discourage their use due to the the quirks you’ve mentioned (and a few others), some organizations flat-out prohibit their use. Here are a few threads on the matter of subtasks:
While it doesn’t directly address your specifics points, you might also find this thread useful: