A couple things: I've never done a red-eye, so I have no advice about the car seat question. However, if your 3.5yo is going to sleep most of the time, I might break my #1 rule and bring it. Airplane seats are VERY small, especially if you're not flying Southwest! Just be prepared for the situation where it's hard for one or both to sleep. Usually the white noise of the plane helps kids, but it never helps me because I'm too focused on my kids and making sure they don't fall off the seat, etc (which might be lessened with the car seat and Ergo).
I would take my own car seats (if you have them). They fly free as checked baggage on most airlines, and I thought pack n plays were in the same boat (baby equipment), but I guess I'm wrong about that. We flew to Edmonton a few months back and took our seat with us (only 1 kid with us) instead of renting. You never know what you're going to get when you rent, or if they'll even have the correct seat. Last time we had to use an airline seat, it was technically illegal for our son to sit in because it was belt-positioning, not harnessed (they were out of the other).
Ergo and stroller are a must for me! I always take my bigger stroller, though, so I don't have to carry the 2 bags we always take with us (or the pillow for that matter). I also always take a pillow with us for nursing and sleeping (I have a 4.5yo and an 18mo, and we fly San Fran to Detroit/Cleveland at least 4 times per year).
I love the idea of craigslist, though! You may even be able to post a want to borrow and see how many responses you get! I also like the Facebook idea :)