I would definitely add the TF for the 3yo as well. Then you can have him RF in one of the center spots. I would put all 3 RF seats in the center spots so that the FF kids could have the tethers. Oldest RF child in the very rear bench since they can help and capable but not necessarily oldest/biggest children on the outboard positions that would be blocked by the RF seats since they need to be able to climb out on their own, especially in an emergency.
They can also help the RF kids get in and out and buckle up if the help is required (you should still check over the harnesses yourself though when you set out, and check for kids getting left behind, I seem to remember a very sad story where everyone assumed that someone else got the smaller child out but no one did...
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BTW are you sure you will have enough room width-wise to install the TFs and two other seats (in one case you will have 2 TFs side by side!)? Have you tried it? The TF is a very wide seat and doesn't puzzle quite as nicely as a Marathon. I hope it will all work for you! Please let us know.
As for safety and crash worthiness and rollover risk, the looks of a vehicle or even the "jumpiness" of it when driving does not always tell the full story. Just the way the 12/15 pass vans are designed (weight distribution and wheel base locations, etc) are just not ideal. That being said, while it's something to consider when you need to buy a new vehicle, you work with what you have. And I am happy to read that you are taking all the necessary steps to have your kids in seats which fit and replacing expired seats. You may be facing some tough choices in the future when the smaller ones no longer RF. Do you think you will be considering driving two separate vehicles to accomodate all the kids?