2) They never explain the mechanics of how people dream together, but Cobb and Mal did the very same thing. There seems to be nothing inherently different about the dream mechanics of entering or leaving the "4th" or subconscious dream state that would cause it to work any differently than 1-3, the defining factor is that it's so deep that in dream time you can be down there for 50+ years for only ~48 hours of being under (at a factor of 12 per level as they state 5 minutes is 1 hour at the first). As they say at the beginning, the mind perceives and creates at the same time without even knowing it's happening, and since it's subconscious it's purely what your mind creates. So after such long periods of time spent there I imagine it's difficult for anyone but the most experienced dreamers still know the difference between that and reality and even fathom that there's a reason they need to "wake up".
That is why people get lost there and it fucks them up in the head so they never know what's real and what's dream after that (Id think some would even spend more time in the dream world than they'd be alive in the real world, hence why they were saying that the dream becomes more of a reality than reality itself). That's why Cobb performed inception on Mal, and he had to deal with the reality that that it wasn't his fault that it was his inception that kept growing that eventually caused her to kill herself even after she woke up... because it's what he had to do to try and get her back. When someone feels guilty, touching on that makes them defensive which is why Mal had always been such an aggressive projection. It was Cobb's subconscious guilt about killing the woman he spent a lifetime with by the inception process that manifested itself throughout the movie as a means to self sabotage their mission to perform inception on Fisher.
Maybe the machine that maintains the sedation also connects the dreamer with some kind of transmitter of their neurological mapping? I didn't bother paying much attention to it because it was clearly something supposedly invented in the future. It could be practically anything.