But you can only ever reroll a die once? You can't reroll with Predator then re-reroll with Han Solo if it comes up blank, for example?

No, you are stuck with that blank (unless you have something that lets you CHANGE a blank/result, not re-roll it)

You can change a die result as much as you like/can, but can only ever re-roll it once.

So for example, you can re-roll a die with Predator, then if it came up as a focus you could use your token to change it (and any other focus results) and you could THEN change one of/multiple hit results into crits if you had the ability.

Got it. Thanks mate!

Han is sometimes matched up with Predator to give you the choice of how many dice to reroll. You roll pretty good but want another shot at one, sometimes two, dice then you use Predator. You don't like most of what you rolled then you just try again with Han. Sometimes you also get the situation where you try to reroll with Predator and after that say 'what the h3ll" and reroll the rest hoping to trigger Gunner so you can try again for better results next time.