FWIW, my wife is fairly petite, 63"/160cm tall, and she was able to fit in the CX-9 third row with the second row seat positioned 1-2" inches forward from its most rearward position. She thought it would be comfortable enough for an hour or so. I'm about 70" tall and average build and my head doesn't fit without slouching. To get my legs to fit, I had to move the second row seat pretty far up and even then my knees were pretty folded up. I could manage back there for a short trip, but it wasn't comfortable and the person in front of me would be short of leg room.3rd row can be used for more than just toddlers.
As for the CX-8's third row, apparently "The third-row seats are purportedly designed so that an adult 170cm (5'7") tall can fit comfortably"