Re: The investigation should center on...
"This is because the planes were deliberately designed to prevent out-of-control engines powering back up, to avoid them causing other problems."
How far out of control?
As far back as WWII, aircraft were provided with the capability to go over 100% rated power if throttles were pushed past a retaining wire. It was called War Emergency Power and, although the engines were either rebuilt or scrapped once this mode was engaged, the idea was that it was better to let the pilots push the aircraft and scrap a couple of engines rather then lose the plane.
With modern aircraft, allowing pilots to exceed a false maximum torque value would be a non issue once the DFDR data was accessed and raw data confirmed operation within limits.