With regard to 6+7.
I think Cooper and Hathaway, entered the Blackhole at the same time. Time slowed for both of them and on earth his daughter went from 30-80 as they slowed. One came out on the other side to the planet, the other fell into the blackhole. This is where I think you have to ignore the fact time would have slowed greater for him in the centre, than her on the outside as it turns out the 5d beings had created a 3d space inside the event horizon.
When he is then pushed out of the hole he is neither part of time or space, as you see from fact he is the person who makes the handshake with Hathaway a few days previous.
None the less he pops out of the wormhole at relatively the same time Hathaway would have made it too the planet, but because of relativity, 50 years had past on earth.
7.. Again, due to relativity, if she had gone to save Hathaway when she was 38, due to the fact Hathaway was on the edge of the blackhole, she'd have had to wait 50 years in the other galaxy until Hathaway appeared again, seemingly to her only minutes would have passed.
Number 2 for me was a bit stupid imo. She said earlier in the film that somewhere what was the solution was to be found in the messages in her room (christ knows why she thought that) on closer inspection she has an epiphany that the STAY message was real, and that the ghost was her father. (again why she jumps to that conclusion I don't know, I assume it's the underlying theme of love jumps dimensions)
She is then going to leave , until she accidentally stumbles upon the watch. Which she recognises, that just like the mavity dust is a binary code. (seems a bit of a stretch but hey)
What annoys me is that, mavity affects the watch were ever it is, not just in the room. As she takes it away, and mavity is not repeating.
I think it begs the question. Were the planets even there too be explored, or was the blackhole the whole reason for the wormhole to that galaxy.
I think the rest have been answered really.