If a Mangler Cannon attack roll is focus-focus-focus and the attacker uses a focus token to convert the focuses to hits, do they then get to change one of the hits to a crit?

If it rolls blank-blank-blank and the attacker uses a reroll and turns one of the blanks to a hit, can that hit them be turned to a crit?

Yes to both

Thanks. What is the rules justification for this?

It's a dice modification effect, and those can be done in any order.