M.B.
Your post sounds identical to what I was worrying about when I got pregnant with my second - and it looks like your kids will be about the same amount of months apart as mine (17 months).
It really is amazing how your LO will adjust to your changing body. At some point it is nearly impossible for him to sit on your lap and lean back comfortably. Ryan sat next to me when I got to that point.
As soon as I found out I was preggo with #2, I made it my mission to get DS1 to fall asleep on his own for naps/nighttime. I am soooo happy we spent the time to do that, because now, with a 20 month old and a 4 month old, I do not have the time to spend 45-60 mins getting DS1 to sleep!
We tried a lot of methods: Cry it Out, "The No Cry Sleep Solution" - which seemed like Cry it Out to me. No "method" worked for us, we did our own thing (gradually got further and further away from his crib until we no longer needed to be in his room for him to fall asleep). It took 6-8 weeks, but was so worth it in the end
Good luck!!