Re: Crib in the bathtub?
The baby is already going to be use to the noise of your home because he/she hears it in the womb. ds1 was so loud as a 2 year old when I was pregnant with ds2, and then ds2 was born and could sleep through ds1's loudness. I wouldn't worry too much about the noise aspect.
I guess I don't see why when you're ready to let baby move to his/her own room before sleeping with the other sibling, why you couldn't just room share with baby for a little longer and then switch to a shared room.