getting killed in the dream without sedatives --> boot you out instantly
getting killed in the dream with sedatives --> your mind ends up in limbo

but now you mentioned it, i think i missed the part where they explained how to get back from the limbo

i think that the young and old is just visual deception, Cobb explained to Ariadne he and his wife lived in the limbo for 50+years...and it was his wife who refused to get back to reality, but Cobb decided that he had enough and want to go back to reality, so he found his wife's safe, and incepted the idea so that they will suicide at the railroad and get back...

however, what Cobb did not expected was the idea he incepted into Mal continues to spread through her conscious and mind(like a virus), and even after she woke up that she still think that she was inside the dream, and in the end she killed herself, and Cobb felt guilty for that act of inception ever since

but i like that ending, his dream had become a reality, so why bother about which state he's in??

they do not confused themselves like what Cobb did,so they don't need to "consult" their totem so often

that the dream is the real world?

i think its because of the different emotions Cobb was in when he told the 2 version of the story:

the young one was when he was still in guilt about the inception he did to Mal, hence he tried to keep the young image

the old one was when he finally managed to let go of Mal

there was a connection actually, remember the first level, where everyone got out from the van, and left Saito, Cobb behind? they were connected through that device

and yah, making limbo state a "shared" place would make explanations much more easier

Cobb was acting as the architect at that moment, so that they could find Mal in the quickest time because Cobb knew where she will be exactly

Arthur did say something about sharing limbo world from someone who had been there previously, and the person is connected to the device with others, so he can share his limbo with those who are connected

for me they purposely used Cobb's world is because it will be the fastest route to find both Fischer and Mal because Cobb knew where both of them will be, and they are running out of time since the synchronized kicks is imminent at that time

well it could stopped, or continued spinning, Nolan deliberately ends it at that moment lol

for Cobb, he got what he wanted and mop away his sense of guilt for his wife, so why care about where did it happens? only thing is us the audiences' brain is messed up trying to figure out lol....maybe, this is Nolan's inception to our mind....lol