Katherine,
I didn't get a chance to read your whole thread or the advice you got yet. But, when DD was born, DS was still sleeping with us. He was 21 months old and for the first few months, we just slept: DH, DS, Me, DD, cosleeper. I was always careful that if I switched her to the other side to nurse to keep my arm between them. I was afraid she would want to be in our bed 100% of the time, but she wasn't like DS and liked having her own space.
Right around his 2nd birthday, we bought DS a twin bed. We got him some cool sheets that he wanted and slowly transitioned him to his bed. It entailed one of us (me, usually) laying with him until he fell asleep, and many little steps leading up to him finally staying in his own bed all night.

I know how intimidating and worrisome it can be. You'll do great! BTW, how's bedrest going?