Sorry bout that, Ryn, the carseat thing was more for the poster who mentioned it. But, as I said, I did it sometimes, myself, cos I knew it didn't happen often and I would check on him all the time. I'd let him sleep in the bouncer if he's happy there - at least they cradle the bum softer than a 'seat', where there is more compaction of the vulnerable spine.
I may have also missed this, but do you have a soft cloth sling? I used to (ha! still do!) get DS to sleep in the sling, and when he was 'convincingly asleep', I'd transfer him to the bed next to me.
Getting DS to sleep requires sling time and a couple of boobie feeds (usually in the sling), but once he's asleep from that, he's out (unless he's sick).
Like someone else said - maybe this requires going back to basics. We did that a few times, each time that DS's patterns changed.
The sling has been our saving grace, though. We used a HAB and now an Ergo, but sometimes a shoulder slilng.
ETA: The sling also keeps baby so close to you that warmth will not be an issue