I feel for you! We've all been there and the sad truth is that all babies do figure it out but its not usually until they are a bit older, especially with your baby's rough start with the RSV.
I've never ever woken a sleeping baby, I think that is doing the baby a great disservice. He will wake when he needs something.
I totally agree with just using a nightlight at night when you are up with him at night and don't talk to him unless absolutely necessary. I'd do the diaper, feed him and back to sleep, all lights out. He will lay there, he doesn't need to be entertained at his age, and he will fall back to sleep eventually.
Your older kids are definitely old enough to work together for you (perhaps bribe them with money or gifts) by watching a movie or playing a game quietly so you can get a nap during the day. Even if you set a timer for 60-90 minutes when the baby sleeps. I'm guessing your 10 year old can be trusted to come wake you if there is an emergency, someone comes to the door, etc. Give yourself the much needed break you need and if you have friends or relatives offering their services by all means, take them up on it! Last time I checked Mom's didn't get any awards for being able to handle it all themselves without help! ha ha.
I wish you lots of luck and more sleep.