Head restraints and top tether routing details aren't indicated for LATCH equipped Galants or Mazda6 models in the LATCH manual. This doesn't necessarily mean these cars don't have specific top tether strap routing requirements in their owner's manuals, though, so you're going to have to double check on that in their manuals to make sure.
Harmony models are not listed in the LATCH manual so I can't post any details about center LATCH or LATCH limits or the carseat's weight by itself for the Defender (it's probably so new that it missed the cutoff for press time for the 2013 LATCH manual).
So you'll need to abide by whatever it states in the Defender manual/labeling regarding LATCH installs, and conservatively assume a limit of 40 lbs. for LATCH installs in the Mazda and assume that center LATCH isn't allowed if the LATCH weight limit and center LATCH allowability is not expressly stated otherwise in the Defender manual/labeling. You can probably find out the Defender's weight on its manufacturer's website to back out its LATCH install limit for the Mitsubishi (65 lbs. total combined weight of your child plus weight of the carseat).
Britax states to assume a 40 lb. lower LATCH anchor install limit if the car manufacturer does not state a numerical limit, as is the case for Mazda models. For abiding by Mitsubishi's LATCH limit of 65 lbs. combined total of the child's weight plus weight of the carseat, the Frontier85 itself weighs 20 lb. according the LATCH manual, allowing use of LATCH until your child weighs 45 lbs. (65-20=45), and you would back out the limit for using the Defender with LATCH in the same manner for the Mitsubishi using the weight of the Defender subtracted from 65 lbs.
Has LATCH got your head spinning as much as mine at this point? So convoluted for a system that was in theory supposed to be simpler, and instead has become even more hugely complicated to sort out in actual practice.
Honestly I'd go straight to trying out seatbelt installs in any of these cars you're considering as you're ultimately going to need to install that way at some point any way whenever the LATCH limit is reached depending on these messy LATCH limits and rules, KWIM?