John S. wrote:
I would worry about the side to side movement strapped to the couch.
Times two!
Whatever seat you place the baby in, it's critically important that the seat be positioned so that the baby faces the rear of the rig- never sideways or toward the front.
Infants that small lack the neck strength to withstand even simple sharp braking events (let alone collision impacts) that force their heads forward or sideways.
The seat into which the car seat is strapped should be front-facing, since the seatback provides
some protection against flying objects that may come loose in such events.