So a die can only be rerolled once, does that count for both attacker and defender? If I use Tarn + R7 to make the attacker reroll his dice, can they no longer be rerolled when its his turn, only modified? He wouldn't be able to use a TL, predator, Scum Boba pilot ability, etc. on those dice?

Correct. Once total.

And kudos for recognizing that modifying dice like with a focus token and rerolling are different things. A lot of new players miss that distinction.

Correct, that's why order of modifying dice is important. The opponent gets to modify first, meaning defender gets to modify attacker's dice first and vice-versa.

So yes, no further rerolling of those dice from any abilities.

Which makes abilities that force rerolls quite handy.