I have a Mazda5 2006 and I'm due in 2 weeks of baby #4.
I have passed multiple hours trying to organize my car seats.
Did you know that the 3rd row has seat belts not really kid's friendly? They are too advanced. Look for threads about mazda5 3rd row booster.
So, the better idea is to put your RF seats in the 3rd row and load your kids by the hatch. It's really easy and my Dd LOVES to be RF there. She says that she sees everything! . I've tried a Radian (with the angle adjuster) and the MyRide. The Radian (with the AA) is perfect: you will be able to place your 2rd row seat on the good spot (no need to advance one or two clicks). But when you'll drive, you won't see a lot in your mirror.
The MyRide is a little bit longer. I had to forward the seat by 2 clicks (that the maximum allowed by Mazda). So it's not optimal but because it's my 2nd car and my childrens use it around once or twice a month, I accept it. Your vision with the mirror is perfect.
So, my dd is riding in a Myride. The new baby will be in a snugride35 in the 3rd row too. I've tried to load the bucket by the hatch but I wasn't able. This bucket is too big. I will have to take off the nbb (harmony literider), then fold the 2nd seat, then load the bucket. It will probably be a little bit difficult and when the baby will grow, it gonna be really heavy to do it. I've read that the Keyfit is easy to load by the hatch. Here, in Canada, the Keyfit has a maximum of 22 pounds. So, I didn't want to buy this bucket.
On the 2nd row, I have one seat FF and one booster. I have bought a nbb instead of hbb to be able to fold easily the 2rd row.
So, in my point of view, the mazda5 is not the ideal car for a family of 4 (our principal car is a Sienna), but for a second car, you can do it.....
Good luck!