Re: Sofa Seat - Adding Additional 3Pt Seat Belt
We have a 3 1/2 year old and a sub-1 yr old, so are pretty much in your same shoes. We've been happy with the Britax so far (I think we have Marathon and Boulevard models). I've added a tether connection point behind the seat to help secure the seat. I don't recall at what weight you are supposed to need the tether, but it certainly makes me feel better having it. That would be a pretty simple modification to add a tether anchor and could be used in the future to secure an 86Y harness.
Speaking of which, I recently ordered (2) 86Y harnesses for future use as well. I haven't installed them yet, but I think they will be a nice option for those of us without 3-pt seatbelts in the rear. I think I will use it with a booster seat that has a back on it first so that it keeps my daughter a bit more contained and would be more comfortable if she fell asleep. The Britax, while a great seat, takes up quite a bit of real estate to pack around in comparison.
2009 Express AWD, CCV Top & 50-ish home build. Daily driver/camper/kid hauler