The language is unclear
The intent may have been that you must wait for your Action to complete before utilizing your Bonus Action.
Jeremy Crawford had previously tweeted that Bonus Action timing was completely up to the player regardless of the trigger requirement. While this gave a lot of freedom of choice, it wasn't really fully in tune with the written rules on Bonus Action timing (emphasis mine).
You choose when to take a bonus action during your turn, unless the bonus action's timing is specified...
However, that tweet has been updated after realizing his mistake.
Clarification about bonus actions: if a feature says you can do X as a bonus action if you do Y, you must do Y before you can do X. For Shield Master, that means the bonus action must come after the Attack action. You decide when it happens afterward that turn.
He further clarified with this tweet
No general rule allows you to insert a bonus action between attacks in a single action. You can interrupt a multiple-attack action with a bonus action/reaction only if the trigger of the bonus action/reaction is an attack, rather than the action.
But that clarification was...wait for it...further clarified again:
"My tweet below was addressing bonus actions and reactions that have triggers. A bonus action that has no trigger—such as Cunning Action and the misty step spell—can take place whenever you want on your turn (PH, 189)
The RAW and the Crawford
It seems clear that Crawford's intent for Bonus Action triggers is that there is a requirement for the triggering Action to fully complete before you can take the Bonus Action. If you go by this, then you are not able to take your Bonus Action inside of (interrupting) your Attack Action.
A question of wording
However, as stated above, GWM is not triggered by the completion of an Action, but by the killing of or reducing an enemy to 0 HP. Given that the trigger is not based on the Attack Action but on the result of a hit combined with the rule on choosing when to take a bonus action during your turn, then it seems possible to interrupt your Action.
Which way to go?
I understand what Crawford is saying in that Actions are complete events that can not be interrupted...except by movement. His clarification that you can't interrupt one action (Attack) with another (Bonus Action) makes some sense, but given the precedent of allowing movement inside of an Action and that the trigger for GWM is not the Action but an event, a GM could rule at their table to allow it.
Crawford's intent here is understandable, but it still doesn't make 100% logical sense given that you can already interrupt your action with movement and/or reactions.
At my table, I generally let my players shift and shuffle actions as long as they aren't actually limited by a clear mechanic. In this case, it makes more than good sense to allow the player this and I haven't seen any issues with it at my tables.
You could certainly Dash first to increase you movement allowance, then attack the first mook, then move, then attack the second mook. That's not exactly what you're asking about though, since you want to see the result of the first attack before deciding to Dash.
In the strict rules as written, however, only movement is explicitly allowed to be broken up arbitrarily. Taking the Attack action doesn't "add to your attack allowance" the way Dash adds to your movement allowance.
Implicitly, when you take an action, you are committed to completing the entire action unless you have a rule that says otherwise. "Moving Between Attacks" is one such rule, and object interaction (PH p.190) is another:
You can also interact with one object or feature of the environment for free, during either your move or your action.
By this reading, you have to complete all the Extra Attacks that are part of the initial Attack action before using Action Surge to Dash.
Best Answer
You can't take the Attack action if your target isn't in range
When you take the Attack action you must immediately make an attack
When you take the Attack action you must immediately make an attack against a valid target.
As Jeremy Crawford has said, there is no gap between taking the Attack action and the attack that results from said action.
If you "take" the Attack action and don't attack, then you have not taken the action. The action and the resulting attack are one and the same.
You must pick a target as soon as you take the Attack action
Under the rules in the PHB under "Making an Attack" it lists the steps one must go through in order to perform an attack.
So, since the enemy is too far away to hit with your melee weapon you cannot choose them as a target for your Attack. However, if you didn't care about being able to attack the enemy, you could attack an object or a location that is within range and that would still be considered an attack for the purposes of Blade Flourish.
After you have completed your Attack, you can move (with extra movement)
No matter what you choose to attack, you can now move with whatever movement you have left plus 10 feet of bonus movement from Blade Flourish.
It is important to note that also that rules only allow you to move during an Attack action if you have multiple attacks. In that specific case, it only allows you to move between the attacks, but not before the first attack. No rule allows someone to move during an Attack action before the first attack.
However, if you are only able to make one attack per turn, then you just used your only attack on an object (for example) which gives you enough movement to reach them, but you have no attacks left to attack them with.
Scenario 1
The Bard can take the attack action to attack a rock nearby. After the attack completes, they can move 40 feet and be within melee range of the enemy. However, since they have already spent their Attack, they cannot attack the enemy this turn.
Scenario 2 & 3
As far as I can tell both of these are exactly the same for the rules that are relevant to Blade Flourish.
The Bard does not get extra movement unless they take the Attack action first. Since the enemy has total cover against the Bard, the Bard cannot attack them from their current position. They can attack an object or something and that will give them extra movement just like Scenario 1.
However, why not just use the Bard's normal movement to go around the corner? Once you see the enemy you can attack them if you are within range. If you attack the enemy (instead of an object/location) then you get the benefits of the extra movement and the extra flourishes the result when you hit a creature.
If the enemy more than 30 feet away however, then the Bard will have to choose between using their Attack to get the bonus movement (and not be able to attack the enemy) or if they want to take another action.
If all you want is extra movement, just use Dash
If you are already spending an action to gain movement and already losing your attack, then you should just take the Dash action and get a lot more movement from that spent action.
Really the true benefit from Blade Flourish comes from when you actually have something within your movement/range that you want to Attack and you get the movement as a bonus to that Attack. It doesn't seem to have much use as an ends to gain more movement when there is already an mechanic made for that very purpose.