I always preface these questions with "I apologize if this has been answered elsewhere" but...

If someone is playing as the Prophet and exhausts the card to help pass an attribute test by subtracting a shield and the overlord plays the card to make them re-roll, do they still get to subtract the shield on the re-roll or is that a one roll benefit?

This lead to a nice long argument over the weekend which almost deadlocked the game.

Thanks!

Yes!

Re-rolls are interesting in Descent, because they do not make a new roll trigger. The shield bonus will persist for the re-roll.

There is an overlord card which actually makes a new roll (not a reroll), "wicked laughter".