I am a software engineer, not an automotive engineer, so my opinions are that of a novice.
1.) I grabbed the tape measure and I see the side arm rests measure 41" across. The Y is roughly 6' (72") across at that point, and the motor and axles are underneath the passengers at that point. So that's 31" total, 31"/2 or 15.5" of crush space on each of the sides. The space to the rear bumper isn't huge, but it seems reasonable; it is less than my Honda Odyssey minivan. A previous version of that minivan was rear ended on a highway and didn't crush in. Given the side and rear cushion, in my estimation, this is safe. I would, however, like it better if there were a side curtain airbag for the rear passengers, and if they extended the bumper out a few inches for 7 seat versions. I don't know if the 7 seater side curtain airbag extends that far.
Net-net, if the kid is in a front facing seat i think he/she would be safe.
2.) My 13 year old is 5' 3.75". (I just measured). He's fine back there but we haven't hit bumpy roads. I'm thinking that the 9 and 11 year olds will be more comfortable, irrespective of that. 13 is pushing the edge of sitting in the way-back. I think of this as an occasional use seat anyway.
If you are anything like me, your kids are at least a couple years apart. The reason the 3rd row Y makes sense for my situation is that my kids are 15,13,11, and 9, and I can count on the way-back working for at least one of them for another 4 years, and the oldest will be out of the house and in college in 2.5 years. It's also the case that we have a minivan (that I DO NOT expect to get rid of anytime soon) that we use for hauling everyone. My Y is for when we are hauling <6, contingency, or short range trips with everyone. I don't expect it to replace the minivan for long trips; my wife is too impatient to stop every 2 hours for charging and there isn't enough space.
The thing is, 99% of our car miles are with less than 6 people in the car at once. So it replaces the minivan for all those trips. Soccer, swim practice, dance, horseback riding, girl scouts, school drop-offs, grocery are all done with 3 people or less. Even ski trips with me and the 3 boys. For the 1% of the time we need room for everyone, or hauling something big, we will use the minivan. Could it work for that 1%? Maybe, but I know my wife wouldn't tolerate it. We could do day trips with it. But if I rewound to when we had 4 kids 0,2,4,and 6, I wouldn't try to replace the minivan with it. Buy a minivan.
HTH!