Thanks so much for the prompt and thoughtful reply. :) I have copied/pasted and replied below!
1. Your oldest child. The seat you want is called a combination seat, not a booster seat. A booster seat is used with the seatbelt only to restrain the child, and is appropriate for a child who has the maturity to sit perfectly still, never messing with the seatbelt, never bending over to pick up a dropped toy, never turning around to identify a source of traffic noise behind the car, etc. The tallest combination seat is the Britax Frontier 85. The harness slots will keep your child safely in a harness until she is ready to convert to booster mode. While not quite as tall, the Graco Nautilus or Graco Argos, and the Recaro Prosport would also fill the need. Your daughter is 40 lbs now, and that is the max weight allowance for the lower anchors in a Honda. Install the seat with the vehicle seatbelt, but also be *Sure* to connect the top tether. The top tether is a crucial part of any forward facing seat and should always be used. This means that the seat should go in one of the outboard captain's chairs. The car seat she was in prior to the booster (which was last week when I pulled it out, lol) went up to 65lbs, but she was just too tall. I will check out the Britax Frontier for her.
2. Your newborn (congrats, by the way!) This is the child I would suggest using the plus-one seat for, because a rear facing seat can be installed with belt only and does not need a top tether anchor. It is also narrow and will fit that silly little plus one base, LOL.
The trick here will be to select a narrow infant seat that doesn't have a base that flares out at the side. This pretty much eliminates the Graco Snugride 35 or the Britax Chaperone, for starters. I think your best bets would be the Chicco Keyfit 30 or the Safety First Onboard Air 35. Of the two, the Onboard Air 35 will last you a lot longer (most kids can get to around 2 in that seat!) When this seat attaches, do I do it with the little lap belt on that plus-one seat? That seat belt just feels so crappy to me.
3. You're not going to like this. Your 1 year old needs to be turned around before you drive in the car again. Vomiting or not. The risk of what could happen to her in an accident FAR exceeds any risk from vomiting. Major head and neck injuries, internal decapitation, paralysis, brain damage, even worse. I would suggest a mirror to alleviate your concerns. Pick a mirror that has soft sides and is made of a flexible material (not one of those big hard plastic ones with toys that plays music). You will be able to see your child just fine. What restraint does your 1 year old currently use? This is one I struggle with. Our ped, OT and GI actually recently (when she hit 30lbs) said to flip her with the choking and vomiting. Every time I would hear her choke and veer out of traffic to try and pull over because I couldn't see her to know she was okay, I thought....there HAS to be a better way to do this, lol. Honestly, what I need to do is never leave home with the 1 yr old with another adult. :) I tried 3 mirrors we had, and due to the car seat itself, and the position she was, I could not get a position to be able to see her in the mirror. (I don't know the car seat brand off hand...DH has that van right now, I will check. It is a 5 pt car seat that goes up to 65 lbs as well). She is rear facing is DHs car because we never go in there without a second adult. I will see if I move her to the other side and redo the mirrors to see her. This disables to ability to reach her quickly when I make a quick stop, if needed, but probably outweighs the benefit/risk. My oldest didn't go front facing until almost 3.25yr because she was pretty light and while she has the same disease as 1 yr old, was better controlled at that point that our youngest now. I will definitely spend some time revisiting this.