This is a hard question to answer, as it primarily depends on the GM's opinion on how death should be reflected in the current game. However, as usual, the GM is not the only one building the ficiton in Dungeon World. The players have just as much inpact on the fiction as the GM, and are even more involved in how things develop than the GM.
Now how does this affect on our possibilities to treat death as a GM?
There are several steps to arrive in a situation where death occurs.
- What decisions lead to a situation where death is likely?
Most importantly: death in Dungeon World (or any RPG for that matter) is not some surprising event that just occurs. There are distinct decisions leading up to a dangerous situation that can cause a PC to die. In this regard, death and the associated Last Breath need not rely on the mechanical fact that a PC drops to 0 HP. If in the fiction you fall off a high cliff, the result is not just d10 damage, because the fall could kill a common person. It is imminent death as a result of some decisions that lead the PC to get too close to the cliff and fall.
The same applies for combat situation. Players are always making decisions. A prime example of PCs facing death during combat scenes is players that just keep fighting on mindlessly against an overwhelming foe. Eventually, those decisions may lead to a PC facing death.
Both situations share an important property: both the GM and the players suspect and fear the imminent death of a PC. Remember, that the GM is required to be a fan of the characters. That implies being worried about the survival of your PCs as much as wanting to provide them an epic death should the situation require. The chain of decisions of the players should give the GM enough time to prepare an epic situation.
- What to do with those actions that lead of to imminent death?
Now as it is in the GM's discretion to use hard or soft moves, the GM has the possibility to artificially set up an epic death. Say, having a PC's head chopped off by the final hit from a foe, which took him to 0 HP. However, such a situation would be, as you stated in your question, extremely hard to narrate and implement mechanically. When exactly should their Last Breath occur? It makes no sense for it to happen after their head is chopped off - how would they survive such a thing?
An alternative to the above situation could be that the PC kept fighting heedlessly and was simply dropped to 0 HP by a standard attack resulting from a missed Hack&Slash, for example. The PC faints and his foe is taking opportunity, preparing the final blow: the perfect moment for a Last Breath! If he succeeds, he briefly enough regains consciousness to avoid the final blow chopping off his head, then falls completely unconcious.
- How would you narrathe this and still keep the looking behind the veil of death part?
The trigger for Last Breath states:
"When you're dying ..."
That is a mechanically very vague trigger and that is good. This way, you can treat the complete process of dying as the trigger. Further:
"The Last Breath is that moment standing between life and death. Time stands still as Death appears to clam the living for his own."
Thus, a good narration could indeed be, that the PC is briefly fixed in time and Death (or your equivalent supernatural force of choice) appears, and, depending on the roll, saves the PC or not.
So the conclusion that this first part provides is that yes, the situation that you initially described in your question indeed can be suitable to handle death.
However, on our quest for epicness, we as GMs often do forget about an important aspect:
Just as in real life, death in the fiction might just not be epic. Even when it is, it is still a bloody mess and a shocking moment for all closely related to the victim.
Another alternative to handle the above situation might invoke less feeling of epicness, but provide a more intense feeling of depression among your group. A PC is usually not alone. Another PC seeing the trouble might intervene. Say, the moment the Fighter faints doe to the lethal attack, the Ranger reacts and sends an arrow to the foe's head, stunning them for a brief moment. Has the Fighter cheated death now? Definitely not, he is still dying to his wounds! In the hands of one of his companion Cleric, he takes his Last Breath, misses and the Cleric feels his souls leave without anything to do. Intense.
Now what is the conclusion of all this? Really, there is no right or wrong. It all depends on the situation, the group, the GM. Anything can be right, and the mechanical trigger of Last Breath allows for a broad interpretation of how death can be dealt with in Dungeon World.