I know you've gotten a lot of replies, but I'll just add my 2¢.
First, all aspects of the 2nd row seats are, as you put it, semi-electrically assisted. There's a bar on the side for reclining and another on the bottom from for forward and backward sliding. These both sorta kinda act like manual mechanical releases, but they aren't. Activating the lever pulls a switch which signals the computer to actuate the release mechanism. Having an electronic release is what allows the hidden button to operate. It's fully electric and basically just releases both the sliding and the reclining locks.
In practice, it doesn't usually matter that the bars are electronic. As with the electronic door handles, they're meant to act like manual releases and you don't notice it most of the time. When I first got my X, I had one defective seat. This is how I realized they were electronic. The seat would lock with the back folded partway down and the two release bars became inoperable. I could pull them all I wanted, but the electronics in the seat were faulty and wouldn't respond to release the seat. Rebooting the car enabled me to use them again a couple of times. This probably isn't an issue anymore, I'm told they've switched to another manufacturer for the seats.
I've got a 4-year-old and an 8-year-old. She was 7 when we bought the X and she always sits in the third row. She has no problem sliding the seats in either direction. However, half the time she doesn't because the gap between the seats is actually wide enough for a small child to climb through without moving the seats (see attached image).
For us the 7-seater is a definite win. We don't often fill all 7 seats, but we do often use the full cargo area, something you don't get in the 6-seater. HIH.