Great questions to ask and address.
The Radian allows bracing but the car manufacturer also has to allow it. Presence or absence of front side torso airbags in the car isn't necessarily an indicator of whether or not bracing is allowable, it's more so how the vehicle manufacturer has arranged its sensors for advanced front airbags overall, and whether or not those sensors can be interfered with by contact with or bracing against the front seat/seatbacks, that ultimately determine it. Your 2010 model is definitely new enough to have advanced front airbags and possibly your 2005 model as well but I don't know specifically whether or not bracing is allowed in either of them.
You'll need to consult the owner's manual for each of the cars, focusing particularly on the airbag and child restraint sections and scouring through those sections thoroughly, to see if there are any restrictions on having a rear facing carseat touching or bracing against either of the front seats.
The short answer on allowability of a center lower LATCH anchor install of the Radian in either of your Mazda6s is no, you need to use the seatbelt. Here's the long-winded, more detailed explanation from the LATCH manual
: While Mazda allows a center lower LATCH anchor installation if the carseat manufacturer allows it with borrowed innermost lower LATCH anchors from each side spaced 18.3" apart in the 2003-2011 Mazda 6, Sunshine Kids/Diono allows it for Radian installations only if the vehicle manufacturer allows and if the spacing between the borrowed lower anchors is between 11 and 14 inches, narrower than the spacing in your Mazda6s. Thus you need to use the vehicle seatbelt for a center installation of the Radian.
The center is safest if you can achieve a proper installation there as it's farther away from either side in any potential side impacts. Either side of the back seat is statistically about equal in terms of side impact crash frequencies if you're not able to successfully install in the center of the back seat. Rear facing in and of itself also is an extra reassuring measure of safety, so go with whichever side works best for your family and driving/parking arrangements if you need to install on the side.
