I don't know how well I can answer that last two part question because.... well, those things actually happened, so you may want to ask someone else how to prevent such things. hahaha.
There were a few key things that contributed to my survival during those first months with a newborn and an almost two-year-old in my care, however. The first thing that I must mention is the Fisher-Price Rock n' Play Sleeper.
It seems so simple, but this thing really saved me- simply because it kept Myer from being able to get too friendly with (read: SIT ON TOP OF) baby Truman. It kept Truman off of the floor, and I could plop that thing down in any room I wanted to be in, and even bring it outside! It kept Truman propped up, which he loved, so he didn't fuss as he would have if I had just laid him flat in a pack n' play or a moses basket. I loved this thing so much because Truman would also sleep really well in that upright position, so he slept in it next to my bed for the first two or three months as well. LIFESAVER.
The next thing that really helped me was my ring sling. If Truman didn't want to be in his rock n' play sleeper, he was in the sling. He loved being cozy in there and it kept my hands free to do whatever I needed to do around the house. It also kept him out of the hands of mischievous Mr. Myer.
When I wanted Truman to have some tummy time, I waited until Myer was napping or I let him do tummy time on his mattress in his crib while Myer and I hung out and played on the floor nearby.
Another helpful routine I found was letting Myer watch a movie or a show while I was making dinner. I could put Truman's rock n' play sleeper in the kitchen while I cooked if he wasn't napping in his crib and cook while keeping my eye on the wee one.
The next tip I have for saving your sanity if you have a newbie and a toddler is to force yourself to get out of the house regularly. Like, at least every other day. Even if it's a walk around the block or a trip to a park or the bookstore or library, do it! It WILL be awkward at first, learning how to manage the kids in a public setting, but keep at it! (I wrote more about that transition here.) If you give up too quickly, your toddler won't have a chance to learn how to behave when you're out of the house and things will never have a chance to start getting easier. You'll get better at feeding the baby while you're on the go and your toddler will benefit from the fresh air and exercise- and so will you!
For me, getting out meant: baby in the sling, with a diaper, wipes, keys, and credit cards in the little sling pocket. Myer would walk (if he could be easily managed wherever we were going), or be in the umbrella stroller (or grocery cart) if I needed him to be more contained. These little outings did MUCH for all of our sanity, and kept us all in better spirits!
Lastly, I must mention how, in our household, maintaining our discipline routines with the older boys is what ultimately kept the clockwork running smoothly. It is so easy to get lax with your older kids once a new baby comes along, but this is really a crucial season to stay on top of correcting your older children's behaviors so that peace and harmony can reign in your house. Myer was testing alllllllll the buttons after I brought Truman home... trying to see what he could get away with... acting out and trying to get all my attention... but when he acted in a way he knew he shouldn't have, I made sure to put the baby down and discipline him in the exact same way I would have before the baby came home. It took a lot of communication and explaining why we don't do certain things to babies, but with consistent loving discipline and communication, there was a lot more peace under our roof!
I hope these quick tips may prove helpful for some of you who are in the thick of that crazy season of life, or who may be approaching it in the near future! Please know you can always feel free to email me with any specific questions you have too - feel free to pick my brain! (my email address is in the right hand column near the top of my blog page.)