Lest you work out anything in advance

Arrggghhh. Here's the issue - 3 kids need carseats. But my original gameplan isn't going to work.

Ruby is in the Evenflo infant seat (which I hate, and should have returned when we bought it for Norah last year, because I'm not 100% happy with how it fits in the car)
Norah and Isaac are both in Britax Marathons.

Isaac was going to go into the Graco turbo booster when Ruby outgrew the Evenflo, and Ruby into the Marathon. Simple, right? Well, Ruby is growing so quick, that she's almost at the end of the weight limit for the Evenflo. It's good to 22 pounds (which is why we picked it, but I still don't like it) She's at 19 and change. She's getting heavy to carry in the bucket. I don't really like bucket seats, but they are so handy until the baby can sit up on their own. So, I could move her into the Marathon rear-facing.

Here's where it gets tricky....Isaac is technically too light for the turbo booster. We have it already, it's in Rob's car. We got a great deal on it, which was nice after the spendy Marathons. But it's for 40 pounds plus. Isaac has topped out at 36 - 37. The turbo boosters in the States are for 30 pounds plus. But Canada's regulations state harnessed seat until 40 pounds. So he's too light. He does use it occasionally when Rob will drive him to preschool in the Camry.

So I don't want to buy a new carseat. I don't want to buy a transitional seat that will, in essence, only be used for a couple of months. The Marathons are good to 48 pounds forward facing, so the girls can stay in them for years. I don't want a car seat that can be used as a booster, because they aren't rated as well as a booster on it's own. If Isaac was big enough for the turbo booster, I could get away without buying a booster until Norah is 48 pounds...and that would take at least 2 years. I like kids in 5 pt harnesses instead of belt-positioning boosters for safety and all, but I want Isaac in that booster already.

So, should I just start stuffing him with ice cream until he hits 40 pounds? Or should I consider him 40 pounds when fully dressed? Rob has pretty much said that he has no desire to spend $140 before tax on a seat that we really won't use more than a couple of months at best. Isaac does seem to be in another growth stage (as seen by his pants all of a sudden being too short). I'm just frustrated....and I want my kids to be safe.