If I have a ship with Sensor Jammers equipped and it is the target of Ten Numb equipped with a Mangler Cannon can I do the following?

Say Ten attacks as rolls only 1 hit. I can use Sensor Jammer to modify it to an eye before they get to change it to a crit correct?

Of course if they had a focus token they could just change it back to a hit and then via the Mangler change that to a crit correct?

right on both counts

defender modifies attack dice before the attacker gets his chance