Really interesting to read this, as i posed a similar question in sleep a couple of days ago. I have a breastfed 5 week old who seems to hate being downstairs in the evening. He fusses and cries all evening. As soon as I take him upstairs he settles and sleeps for 4-5 hours! (I have a toddler who didn't sleep that long till she was 2, so I'm amazed and chuffed I've got a decent sleeper... [for now, i don't doubt it'll change]).
I will start a routine, as I did for DD at around 3 months, which will involve him being alone upstairs for a couple of hours. Obviously will check on him frequently. If DH or I sat upstairs with him then we'd never see each other, DH doesn't get home till 7 and we cook and eat after that.
I find the advice to always be in the same room completely impractical. I'm usually in the same room as him while he's sleeping, but if i need to move away to make a brew/lunch/answer the door/hang the laundry, etc etc etc, then i will. If the toddler needs her bum wiping or is about to do something dangerous then of course I'll have to leave him. If he's asleep when I'm doing the toddlers bedtime routine then there's no chance in hell I'll move him into the bathroom when she's having a bath, or into her room while I'm settling her, and risk him waking up and throwing her bedtime off. I just put him down and pray he stays asleep! Similarly, in the morning we leave him sleeping so that we can both shower, dress, dress and feed toddler, etc, and pray he stays asleep. I don’t understand how the people who never leave their babies sleeping get on with anything else??? Especially if they have other kids?
Interesting about the theory being that they regulate their breathing from hearing you breathe. My eldest is profoundly deaf from birth, so wouldn't have ever heard us breathe. I wonder if any retrospective sids studies have been done looking at association with hearing loss.