I believe a single die may only be re-rolled once.

How about dice modification, once a die has been modified can the same player either re-roll or modify this die again?

If you mean "Can I, with a Mangler cannon and a focus token go from Focus-Focus-Focus to Hit-Hit-Hit to Hit-Hit-Crit", then, yes, absolutely you can.

Yes, you can modify a die more than once. Yes, you can modify a die that has been rerolled. Yes, you can reroll a die that has been modified. The ONLY limitation is that you can't reroll a die that has been rerolled (even if other modifications happened to it in-between).

Also don't forget special effects that limit what you can do e.g. Sensor Jammer specifically prevents re-rolling the affected die after modifying it

and the timing of who modifies when usually comes up for order

Also don't forget special effects that limit what you can do e.g. Sensor Jammer specifically prevents re-rolling the affected die after modifying it

and the timing of who modifies when usually comes up for order

Thus my question: if I modify a hit to an eyeball with Sensor Jammer, can it then be modified back to a hit if the attacker has a focus token?

Yes, they can. They can also abilities such as Farlander's or Poe's.

Before someone asks, the point of the Sensor Jammer is that the opponent has to have that Focus or ability available to turn the dice back. If they take a different action, lose their action to an overlap, or keep/spend the Focus token for defence, they're losing damage. It rather severely limits your opponent's options.

I am confused.

My understanding of the sequence:

1) Roll dice (in this example, attacking)

2) Defender modifies and/or forces re-rolls to Attacker's rolled dice.

3) Attacker modifies and/or re-rolls any of the Attack dice (subject to special abilities used/possessed by the defender, and with the caveat that if a die was re-rolled in step 2, it cannot be re-rolled again in step 3 under any circumstances.

And as I think of it, step 4

4) Attacker may 'cancel' his dice by instruction of special cards. These cards instruct what the player substitutes for his rolled dice (see Accuracy Corrector for example).

I call it step 4 because it has confused a few people when in the sequence of event "cancelling" dice happens. It happens after all results are finalized.

A defender doesn't get to go back and forth undoing what the attacker is changing.

Defender MUST use force all re-rolls and modify all dice before the attacker. Once he says he is done, he is done.

So what are you still confused about? You seem to have the proper procedure.

I am confused.

My understanding of the sequence:

1) Roll dice (in this example, attacking)

2) Defender modifies and/or forces re-rolls to Attacker's rolled dice.

3) Attacker modifies and/or re-rolls any of the Attack dice (subject to special abilities used/possessed by the defender, and with the caveat that if a die was re-rolled in step 2, it cannot be re-rolled again in step 3 under any circumstances.

Your caveat in step 3 is too specific. The rule isn't "both players can't reroll the same die". It's "no die can be rerolled more than once", regardless of whether it'd be the same or a different player doing it the second time.

And as I think of it, step 4

4) Attacker may 'cancel' his dice by instruction of special cards. These cards instruct what the player substitutes for his rolled dice (see Accuracy Corrector for example).

I call it step 4 because it has confused a few people when in the sequence of event "cancelling" dice happens. It happens after all results are finalized.

Accuracy Corrector is not used after finalizing results. It's used during your step 3 by explicit errata [FAQ 3.21 & earlier].

Things like Ion Cannons, Twin Laser Turrets, and Bossk (Pilot) cancel dice after results are finalized because they specifically only work if you hit, and you don't know if you've hit until everyone's done rolling & adjusting everything. It's not because they use the word "cancel".

A defender doesn't get to go back and forth undoing what the attacker is changing.

Defender MUST use force all re-rolls and modify all dice before the attacker. Once he says he is done, he is done.