Thanks for your response. My concern is less about the seatbelts, but rather about the positioning of the child seat given the "bucket" nature of the rear seat. (See the first picture below with the red part outlining what I'm calling the "bucket").
Due to the offset towards the middle required to clear the upper Celica
convertible mechanisms (see the blue arrow in the pic above), most, if not all child seat bases will not fit inside the "bucket". Therefore if I have to put part of the base on the side of the "bucket" of the Celica seat (say near the speaker grill in the above picture, or near the seatbelts in the above picture, so that part of the child seat is on the Celica "bucket" surrounding), and part of the child base on a rolled towel or pool floaty inside the bucket to essentially level it off with the side of the "bucket", it doesn't seem like it is going to end up too stable. (See pic below with the tan monster representing a child seat).
Therefore part of my thinking is if I install and secure thick plywood over the bucket on the sides of the seat (see figure below), that provides a uniform flat surface to set the seat on but of course still mount it according to specifications with the Celica seatbelts and tether from the car seat if applicable, both stable and slightly offset from the center of the bucket to clear the Celica
convertible mechanisms. (see final picture).
Does that make any more sense?