have you considered a battery warmer wrap? size it to warm battery to optimum temp in the time the pan heats up. it may do nothing but cheap to try.
you might want to make sure those rear windows don't filter UV or any other spectrum like almost all car windows do now.
i know my old '93 caddy doesn't as it heats up fast in dead of winter with sunlight and no heat running - not so awesome in the summer though.
a vertical mount is probably better than horizontal for most of the year.
it depends on the type of converter you use, if it has MPPT or not, and how the cells are wired and deal with shading. if you use the same small inverter for both panels (it will likely only have one input or combine them inside the unit), you'll have to keep both windows in the sun, otherwise the shaded panel will kill the output as the inverter can't cope with that on a single input.
'maybe' and i am doubtful, but you could get away without an inverter with the right type of panels and just wiring direct with that small of size. as long as you have a blocking diode on them and shut them off while you recharge at home (so you don't overcharge or mess with the chargers detection circuitry)
you can't overcharge while driving as you'll use at least that many watts. if the panel voltage ranges are close to vehicle specs, the vehicle should cope; it probably won't care about low voltage. your cpu might trigger a fault code. i don't think the expense is worth it with an inverter, unless you drove long trips and parked shortly. you'll have to cut the panels when parked for any length of time as you can't control the charging.
my little panel is safe as it doesn't put out enough juice to effect anything in a possible overcharge state. plus i tested my car's usage when parked and it's about the same as max solar output. unfortunately the engineers didn't seem to care about the drain in a big car that normally has a gigantic battery. if i keep the radio constant power on (stores settings) then it doubles. i scored it for $10 bucks open box so it was cost effective. I removed the frame and it's barely noticeable on the dash; i had it on the visor flipped down, but passengers got annoyed flipping it back and the wiring connection to the circuit board is fragile without the frame. the dimming strip across the top of the window also cut the solar input so i had to flip it vertical and i would get constant looks like it was a gang symbol or something.
do you have a link to your changes to reduce your power usage?
i was planning on trying the alt delete with a switch as my alternator comes with a kill wire; you just ground it so no hacks needed. even has a nice male plug blade connector on it. the service manual has many tests the require the alt to be killed so i guess back then they included as a courtesy to the techs.