Okay, I've figured out why Car 53 is reported as 15th in the official result and not in-game. In the rush to get the race back on, my decision to get the race back on instead of spending
all the time necessary to get the grid accurate and know what was going on in my head, I erroneously presumed everyone was on the leader's lap except Car 75. In reality,
as the tracker shows at the rollback point, Cars 53 and 61 were a lap behind the GT2 leader, and Car 75 was two laps behind the GT2 leader. These gaps will need to be taken into account when looking at positions at the lap completed at halfway.
I am, at this time, confident that the results are accurate, and that in-game was not trustworthy at any point in the restarted race.