The Standard Action - Ranged Weapons attack - is resolved only after the damage roll (if a hit was achieved) was made and all Momentum spends regarding this damage roll (re-roll of damage dice, penetration spend, etc.) were made.
Then, the action is fully resolved.
Any Momentum from that action, that is still unspent, can be saved in the group Momentum pool, but only to the maximum of 6 Momentum. Any excess is lost.
Momentum does not disappear instantly, only after the task is fully resolved.
That is the situation for the Swift Attack Momentum spend during a combat scene.
It is your character’s turn, you make your Standard Action as a Ranged Combat attack, generating a lot of Momentum.
The attack will be fully resolved - including the damage roll and any Momentum spends regarding that.
The remaining “floating” Momentum is still your personal amount of Momentum during your turn, it is not (yet) stored in the group Momentum pool.
If you immediately after your Standard Action spend 2 Momentum for the Swift Attack, you can draw that from your “floating” Momentum. - But you could end your turn here and, for example, make the Swift Attack on a Reaction later in the round, so not on your actual turn, but on the turn of the attacker you made the Reaction against. In that case, the Momentum for the Swift Attack comes from the Reaction - Acrobatics or Parry test - as “floating” Momentum or from the group Momentum pool.
The point where it gets weird is, when the archer in your example has 3 Momentum left after declaring the Swift Attack and buying another d20 for Momentum.
In that case, he starts another action, which could be another Ranged Weapons attack, but it could be a totally different Skill test which requires a Standard Action.
The 3 Momentum he has left will be stored in the group Momentum pool, before he makes the dice rolls for the next attack. They don’t carry over into the next attack.
This is important, as with Riposte Talent, they DO carry over into the Riposte attack - which is an exception.
As there were 2 Momentum in the group pool before he made the first attack, the group pool is now at 5 Momentum before he rolls for the second attack. This generates 4 Momentum, 3 of which he spends for Confidence, and the last point goes into the group pool, maxing it out at 6 Momentum.
Momentum is, unless a Talent says otherwise, only “floating”, until the action is resolved and you made all your spends. When you undertake another action, any surplus, unspent Momentum goes to the group pool or is dropped if the pool is maxed out.
You only have a single Minor Action per round.
In your example the archer used THREE of them:
First Minor Action to use Marksman for 1 Momentum (which you allow him to spend before the actual Standard Action).
The second Minor Action to use Quick Release for spending 2 Loads on the shot.
The third Minor Action to use Quick Release again for the second attack.
This makes three Minor actions, which is not possible according to the combat rules.
If your archer used the Marksman Talent, then this would have used the single allowed Minor Action, so he would have only been able to spend 1 Load on his shot, maybe buying additional d20 for group Momentum or Doom, and still rolling 1 damage die less than in your example.
And the same on the second attack.
If you allow your player to use any number of Minor Actions, then they often don’t need to generate Doom or use group Momentum. So you get this very “effective” archer as a generator of group Momentum.)