as far as I know (though anyone who knows differently please chime in) there isn't a safety difference between properly used latch and a proper seatbelt install. but, my kids carseats don't fit next to each other, so I have them both outboard, the toddler behind the driver and the baby behind the passenger because to fit a seat behind the driver we have to have it less reclined
we have a matrix too-I have had the kids in a variety of positions, currently with dd (FF) passenger outboard and ds (RF) drivers's side outboard. I had dd in the center for a while and will probably move ds in the center when we head out on a long drive for vacation so I can sit next to him. I think that, while the center is safest, the outboard seats are certainly not unsafe. I don't believe that one side is significantly safter than the other, though I could be wrong?
Also, I couldn't sit comfortably between two car seats in the back-I fit, but just barely, so if you are planning on sitting back there a lot, I would try to get one car seat in the middle. (I now have a radian and a matrix back there and can just barely get them next to each other.)
Oh, and just FYI, because I didn't know this until somebody on this board kindly filled me in, my 2007 matrix has locking seatbelts, so seatbelt install was actually really quick and easy and just as safe as using LATCH. Both of my seats are currently installed with seatbelts to give me some more room :)
Things to consider:
1. There is no difference in safety between a convertible and an infant bucket.
2. There is no difference in safety between a seatbelt and a LATCH install.
3. If you sit in the back, you need to have head rest support to the tops of your ears. Is there an adjustable vehicle head rest in the middle seat? If not, you need to either see if the infant carrier and Coccorro can be installed next to each other so you can use an outboard seating position, or you'll need to sit in the front passenger.
4. The Baby Trend infant seat MUST have its handle bar in the 'down' position in the car. Make sure that you can do this and still have the adult sitting in front safely pushed back from the airbag/dashboard. If not, you may want to consider a different seat that allows the handlebar to be upright, OR put your newborn in the Coccorro and buy a larger rear facing seat for your two year old. The two year old, unless he is very tiny, is going to need a larger rear facing convertible very soon anyway.