What model boat and what model motors?
It is difficult to see the cracks in some of your photos but based on what I can see, he transom is flexing. The second pic is the more worrisome since behind that area on the port side is where a stringer bonds to the transom. Although I dislike the climb on the motor lower unit and bounce up and down, in this case that stressing may help you determine how much flex the transom is experiencing. If you visible see the crack opening or closing as you stress the rear, that is not a good sign. Are there any stress cracks on the outside of the transom?
You probably would benefit by having a fiberglass person take a look, tap around a bit and look for any evidence of a soft/wet transom core.