We have the 2005 Civic and nothing fits 3 across. I have not heard 3 Radians even do, at least not confirmed it. I did have the Frontier85 in this car, and I liked it just fine. It does recline back a bit due to the angle of the Civic itself, and you can't just adjust the harness slots once it's installed. You need to go behind to do that, so that would mean uninstalling the seat, more or less. So don't look at that easy adjust as a huge feature. It just means you don't have to re-thread. (I know, I was hoping for an easy feature there, too).
I don't think weight will matter, as they will outgrow by height, most likely. You might consider too if you get a good install.
Honda does not allow using Latch beyond 40 lbs, so you'll have to do seatbelt install in the Civic after 40 lbs. Make sure you get a good install. And the new Super Latch feature would not apply to your 2004, so please don't use latch on this seat.
But as long as the straps are ABOVE the shoulders for forward-facing, any kids can use it who meets the seat requirements. For example, my 4 and 6-year-old can use the same seat without any harness slot adjustments.
We too want to harness as long as possible. My kids are all in Radians and the oldest is 6. I probably should have gotten the Frontier85 for longevity. I want to harness until 8. But at the time we needed 3 across a Malibu. So yes, the Frontier is big, but if you do 2 in a row it doesn't really matter. I would get the SICT version with the enhanced side-impact protection, too. Again, 2 in a row anything fits.
The Radian has the Safe-stop feature. So I found it an awkward seat if you have a kid under 40 lbs (requires Safe-stop) and one over 40 lbs. (you need to unhook it), both using the same seat.
Again, I think it comes down to fit in the vehicle. I actually want to get a Frontier85SICT for my 6-year-old soon. Britax is getting rid of harmful flame retardants by the end of 2012 too, they claim. But the Radian seats have always tested well for toxins, so no major worries anyway.
