I have a clek. I've tried it in the middle with the latch and found it a bit wobbly since the middle is raise. It's more wobbly than in my Corolla. But my passenger seat up against it kept it stable. When in any of the other back seats, it was perfectly stable. However, If you have a front seat right up against the rear facing clek, I've found the clek to scratch the plastic part behind the font seat. As far as passengers sitting around, I've found the base is fine since it's narrower there but if your passengers have a wide frame, they might be resting their arms on the car seat as the upper part widens right where my arms would want to rest. I have not tried front facing but I think that would feel even tighter since the widest area goes all the way up to the shoulders.
All in all, I'm really happy with my clek and bought it similarly for the narrower width as I drive other passengers for longer drive vacations. So it's me with my wife in the back, baby in the middle, friend (wife) in the back, friend (husband) in the front and their dog in the middle floor. If you are in the position to have 3 kids that all need rear/front facing it's a no brainer to get something narrower.