The comments that i found about fitting the seat in the middle were that if you could push down on the child-seat and compress the cushion enough to get the seat to sit flat,belted in tightly so that it doesn’t move around (on other subaru forums). The Middle position isn’t always the easiest to install a car seat which is why Subaru says to use the latch anchors which are rarely in the middle.
The manual gives you two options, the car is designed to secure two child-seats with the Latch system and there is a procedure mentioned if the middle position is preferred. If it were possible/advisable to use the Latch mounts from either side in the middle they would have mentioned it in the manual.