First off...congrats on the new little one! I'm having a repeat c-section Dec 28th...8 weeks to go too!
We've never had trouble keeping Mara (she'll be 3 the 22nd)
We had very similar problems with our Son. It got so out of control, that we would be up for hours each night just trying to get him to stay in bed. We went and saw a behavorist, because I couldn't take it anymore.
Her advice was to have a bedtime routine (ie, bath, book, lights out, kisses, bed) which we already did, stick to the same time each night, which we already did and make it clear it is not a neogatition. She said do the routine, say goodnight and he is to stay in bed, and expect him to do it. Then each time he gets out, pick him up and put him back into bed. Which we started doing, this led to tanturms....after about 5 or 6 VERY LONG nights, no more problems.
Sounds very Super Nanny, I know, but it did work.