So let's say horton uses a prton Torp and rolls two blanks a focus and a hit. he rerolls the blanks and gets a focus and a blank. does he gain reroll that new blank?

No. You can't reroll the same die more than once.

Horton is great with Proton Torpedos. I put two of them on him in my build.

Ya ive never really used horton becaus eive always used ywings for the ion turret. but i wanted to mess around with the new astromech that give ywings an EPT. so i made a build with wes+VI and wedge with opportunist (wes fires first strips a token then wedge blasts away with opportunist) horton with flachette, proton astromech and PTL. that way i could focus and TL then hopefully get the kill shot after wes and wedge do their thing. thanks!