My question is whether Davith can trigger his Force Speed ability after his normal attack resolves but before the effects of Fell Swoop begin to resolve. Here are the relevant rules I could find on the subject, seems to me that there's nothing to prevent Davith from using Force Speed before the free attack from Fell Swoop, allowing him to effectively move 4 spaces before performing the free attack. Note that this is different from Biv's Close & Personal as the attack from Fell Swoop occurs after the first attack (and thus action) has fully resolved,

Force Speed - 1 strain: Use during your activation to move up to 2 spaces. Limit once per activation.

Fell Swoop - surge+surge: After this attack resolves, become Hidden, move up to 2 spaces, then perform another attack.

Abilities: An ability that can be used “during your activation” can be used before or after performing an action during an activation. It cannot be used while resolving an action.

Conflicts: When Rebel players wish to resolve multiple effects at the same time, they collectively decide the order of resolution.

Reading through the rules further, it looks like there is also no restriction on spending movement points or using strain to gain movement points between resolving an attack and triggering an ability that performs an attack. Note that Fell Swoop is not an interrupt. Am I missing a key rule here?

Movement Points - Spending movement points is not an action and may be done at any time during the figure’s activation. They can be spent before or after performing any action but cannot be spent while resolving an action.

Strain - A hero can suffer 1C (strain) at any point during his activation to gain one movement point. He may do this up to twice per activation

As soon as you pay the two surges for Fell Swoop, you have triggered another action.

Because you are resolving this new action you cannot trigger an ability, such as Force Speed.

Essentially;

Attack, gaining 2 surges and spending them on Fell Swoop.

Immediately following resolution of Attack, perform Fell Swoop.

There isn't a gap between the resolution of the first action and the trigger for the second action to use the ability.

As soon as you pay the two surges for Fell Swoop, you have triggered another action.

Because you are resolving this new action you cannot trigger an ability, such as Force Speed.

Essentially;

Attack, gaining 2 surges and spending them on Fell Swoop.

Immediately following resolution of Attack, perform Fell Swoop.

There isn't a gap between the resolution of the first action and the trigger for the second action to use the ability.

To add to this, it's not actually another action... it's still the same action.

The action doesn't finish until you complete both the original attack and the fell swoop attack.

Similar to Brutality. It's two attacks in the same action, not two separate attack actions.

Brutality doesn't use the phrase, "after this attack resolves". Come to think of it, I couldn't find anything in rulebook regarding what that phrase even means. Is it defined in the rules or just inferred?

I think I get what you're saying about there being a difference between resolving an action and resolving an attack, I just haven't found a clear explanation in the rules.

Fell Swoop (class card) is resolved when the attack resolves. "After the attack resolves" is a trigger, it does not mean at any time during the activation after attacking.

Unless you have other abilities with the same trigger, which fall under the timing rules of simultaneous triggers, if you do anything else (like suffering strain for movement points, spending movement points, performing an ability with different trigger, including performing another action), you forego the trigger of "after attack resolves" and can no longer perform Fell Swoop (or other after attack resolves abilities).

See a list of abilities related to attacks and their timing from this article: https://www.boardgamegeek.com/article/22915112#22915112

Brutality doesn't use the phrase, "after this attack resolves". Come to think of it, I couldn't find anything in rulebook regarding what that phrase even means. Is it defined in the rules or just inferred?

I think I get what you're saying about there being a difference between resolving an action and resolving an attack, I just haven't found a clear explanation in the rules.

What's Brutality got to do with this? Brutality lets you perform two attacks. You cannot suffer strain for movement points or spend movement points during an action (or perform any other "during your activation abilities"), thus you cannot spend movement points between the two attacks of Brutality.

It's a different than "after attack resolves" abilities.

Similar question on fell swoop, it says 2 spaces, so could you move thru an adjacent enemy and trigger blindside?

Yes. Blindside can be triggered while moving those 2 spaces from Fell Swoop.