I'm unsure of the timing between a particular interaction in campaign.

The Imperial Player has Experimental Arms on a figure (may add 1 surge to the attack. If he does, he takes 1 damage after the attack is resolved). Suppose the figure has "Surge: +2 attack" as an inherent ability.

A Rebel player has Combat Coat, the relevant portion being "Exhaust this card to convert 1 evade to 1 block"

Suppose now the Experimental Arms figure attacks the Rebel, and the result is 3 attack (no surges). The Rebel rolls 1 evade. All else equal, the Rebel wants to use the Combat Coat, since the evade is currently useless. However, he knows that if Experimental Arms were used, it's better to have that as an evade.

Does the Rebel need not decide whether to use Combat Coat until after the opportunity to invoke Experimental Arms has passed? Who gets "priority" here?

Relevant rules:

While attacking, mission rules are resolved first, followed by effects from the attacker (including figures friendly to him), then effects from the defender (including figures friendly to him).

(Steps 4 & 5 of an attack):

4. Apply Modifiers: If players have any effects that gain or remove icons or Accuracy, they are applied at this time. This includes spending F (evade) results to remove B results. Any B abilities that provide modifiers are not resolved until step 5.

5. Spend Surges: If the attacker has any B results, he may spend them to trigger special abilities.

So, do the decisions have to happen latest in step 4? That is, in step 4, we ask the attacker "Would you like to use Experimental Arms?", then, afterwards, the defender is asked "Would you like to use Combat Coat"?

I would rule this as the attacker has to go first, then the defender, and you can't go back and use Experimental Arms based on what the defender did.

Step 4 seems to be where you add or remove any modifiers to the results. Both Experimental Arms and Combat Coat are things that modify the results, so they should go there.

And inside step 4, it is attacker then defender.

There is a similar situation when you reroll dice.

The attacker has to reroll dice first if he wants, then the defender gets to decide if he wants to reroll any dice. If the defender gets a good reroll, the attacker can't go back and use his reroll.

If I understand your setup correctly, I'd say that you're interpretation in the sequence of events is correct. As the attacker, the Imperial player resolves any of their effects first, followed by the Rebel player.

Attack Steps (skipping to relevant bits):

- Step 4: Apply Modifiers - As "Experimental Arms" adds a Surge icon to the attack result, the Imperial player must decide whether or not to do so at this time. Additionally, the Rebel player must decide whether or not to use "Combat Coat" at this time.

- Step 5: Spend Surges - If "Experimental Arms" was declared, the Imperial player triggers any of their appropriate surge effects now, such as the +2 Damage surge you suggested above.

Relevant RRG Pages (Emphasis mine):

- Page 2: Timing - "During an attack, in both a campaign and a skirmish, resolve mission rules first, followed by effects triggered by the attacker (Imperial) , then effects triggered by the defender (Rebel) ."

If I understand your setup correctly, I'd say that you're interpretation in the sequence of events is correct. As the attacker, the Imperial player resolves any of their effects first, followed by the Rebel player.

Attack Steps (skipping to relevant bits):

- Step 4: Apply Modifiers - As "Experimental Arms" adds a Surge icon to the attack result, the Imperial player must decide whether or not to do so at this time. Additionally, the Rebel player must decide whether or not to use "Combat Coat" at this time.

- Step 5: Spend Surges - If "Experimental Arms" was declared, the Imperial player triggers any of their appropriate surge effects now, such as the +2 Damage surge you suggested above.

Relevant RRG Pages (Emphasis mine):

- Page 2: Timing - "During an attack, in both a campaign and a skirmish, resolve mission rules first, followed by effects triggered by the attacker (Imperial) , then effects triggered by the defender (Rebel) ."

The top is correct, Combat Coat and Experimental Arms arms are both used in Step 4.

So Combat Coat must be used before Rebels figure out what the surge is used for (of course rollback is allowed if the IP player doesn't prompt them allowing to use it, within reasonable timing).

As for your bottom, during an attack, conflicts are different:

While attacking, mission rules are resolved first, followed by effects from the attacker (including figures friendly to him), then effects from the defender (including figures friendly to him).

• Effects from multiple figures under the control of a single player are resolved in the order of that player’s choice.

• The decision for a player to resolve an optional effect coincides with the order of resolution. After a player has declined to trigger one or more abilities, he does not have the opportunity to trigger that ability until the next triggering instance.

tl;dr: Attacker activates abilities first. During the Rebels attack, the Rebels activate bonus surge abilities first (or if they respond to the IPs defense ability, the IP has a right to rescind it).