My 5 was a 2008.
There have been some changes since then, but nothing too incredibly major, from what I understand.
There are adjustable/removeable headrests in *all* seating positions. In my model, the headrests had to be down for installation, but that was redesigned in 2010, I believe, and you no longer need to.
The 2nd and 3rd row seats fold flat which creates an insane amount of cargo, should you need it. The 3rd row is a 50/50 split.
You could definitely fit 4 carseats, depending on what they are. The seats and seatbelts, I thought, were very carseat friendly. I never had an installation issue. I had issues with the 3rd row seatbelts with a high back booster, but again, my understanding is that Mazda redesigned those 3rd row seatbelts, so it may be a non-issue now. You could put your RF'ers in the middle row and your FF'ers in the 3rd row, that would, probably, take up the least amount of space. There is not a huge amount of legroom in the 3rd row, but it's definitely do-able, especially for a sometimes kind of thing.
There are top tethers for each rear seating position. Lower anchors in the middle row captain's chairs only.
I absolutely loved my 5. It has been my favorite car EVER. I, literally, cried when we traded it in for a fullsize minivan. But...such is life.
Good luck!