So i now that you can reroll dice only once. Since the defense modifies dice first during an atack: Lets say Tarn has an R7 unit. he is at range 1 of Scum Boba. Tarn spends the TL to make him reroll a die. It ends up being a blank. Can Boba use either his ability or a TL to reroll the die Tarn already made him reroll? OR does the no re-rolling a die more than once apply?

You cannot re-roll the die more than once.
That's one of the reasons R7 is powerful.

As DraconPyrothayan already notes you can only reroll a given die once. If the Defense forces the attack die reroll first the Attacker can't do anything about rolling but CAN use any other modification abilities on it.

If Tarn w/ R7 rerolls a [kaboom] result and that ends up as an [eye] result the Attacker may still have abilities that could turn that [eye] into a [boom] or even a [kaboom]. Depending on what is used even a [blank] result could be turned by an attacker to the point it ends up as a [kaboom] but it could still only be rerolled one time.

Does Scum Boba cause a reroll before the TL kicks in? I thought the order went attacker rolls, defender modifies dice (I assume forcing a reroll is a modificatio), attacker modifies dice (insert TL here), then defender roll evade dice.

Am I inserting the TL incorrectly here?

Forcing the reroll is a modification, and any die that has been rerolled can't be rerolled again, by either player. It's best to set that die aside so you both know that it's showing it's final result.