I don't disagree, but you'd think Heimdall would have noticed at least. I mean that's his job, no? Those Kree must have been *very* sneaky.
He may have. But his duty is to protect Asgard, not Midgard.
I know from reading some comics as well as watching various cartoon adaptations that in Earth-616, Asgard and the other realms are indeed supposed to be other dimensions. However, I'm confused as to the intent in the MCU because in Thor, Thor used Hubble snapshots to explain the Nine Realms to Jane Foster.
That and the fact that the Elves seemed to travel by something closer to conventional space travel makes things more confusing. However, it's honestly not something worth overthinking. The Nine Realms are unique because there are nine of them. They're different from the hundreds of thousands of habitable planets in the universe. I wouldn't think about it any more than that.