Idk, I'm using back seats in BRZ quite regularly for 1 day trips with the same group of friends and it's pretty fine. Please consider we Europeans are smaller people in general. I'm like 5.5ft dwarf for example and some of my friends are even shorter so I don't even need to move my seat forward when someone sits behind me xD
It won't be that long trip - we're booking hotel for few days so it's not like we're going to spend all that much time in car. Probably only around 4h trip to the hotel including breaks and then another 4h going back. If we'd spend fewer days in hotel it would be no problem at all but since we're gonna stay here for a while everyone needs some clothes to change etc etc. That's why it's mostly problematic in terms of cargo space.
So like I said - we've been there, done that. It's fine. It's much bigger than my MX5 NB and I also never had issues with cargo space in MX5.
I think people are over dramatizing minor inconveniences...