Sending will fail (but ask your DM)
Trying to reach Snurgle: the Nurgle Aunt Amalia knew does not exist currently, so her Sending will fail.
Trying to reach Benedict: the Benedict Aunt Amalia knew likewise does not exist currently, so her Sending will fail. It seems poor Amalia will have a hard time to get the components she wants.
Waldemar interacted with a creature that is Snurgle's soul in Benedict's body. Even though he does not know that this is not the real Benedict, that is the creature he knows, and his Sending to it will succeed.
That said, there is a lot of ambiguity around creatures, souls, and what it means to be familiar with a creature, so this is an area where each DM has a lot of freedom to make up their own interpretation and rule how they like.
Here is the rationale for the interpretation presented above:
What is a creature?
A creature is a soul with a body, not a soul by itself, nor a body by itself. We know so, because DMG p. 23 says:
When a creature dies, its soul departs its body
In the case of constructs, it instead often is a spirit bound into a constructed body, see for example the lore of Golem or Scarecrow. In the case of a Ghost or Specter, it is a soul or spirit manifested as an (incorporeal) body, see their lore.° In any case, a creature is the total of body and whatever soul or spirit it might have or be at the time, not any part thereof.
If the soul leaves its original body, that original creature ceases to be that creature. If a creature's body or soul are not the ones it originally started out with, the resulting new combination of body and soul make up a new creature, too.
If you try to contact a creature with Sending, and that creature in its combination of soul and body does not currently exist, the spell will fail.
Being familiar with the creature
Familiarity with a creature is not clearly defined but whatever familiarity exactly means, we can say that you are familiar with the creature you interacted with. If the combination of body and soul was different at that time than from what they are now, that creature does not currently exist any more. As a consequence, when you try to reach them with Sending, the spell will fail.
This Q&A discusses creatures not being what they appear to be, and the consensus answer is that you will contact who you think you are contacting. If the creature you are familiar with does not exist any more -- in one of the examples it has been replaced by a doppelganger -- the conclusion is that the spell fails°°.
Downsides of this reading
The approach used here has the upside that it gives an umambiguous and clean interepretation that is supported by what the game says about the nature of creatures. It also has downsides:
In the course of a campaign it may well happen that a player character's body gets replaced. In the Dice, Camera, Action campaign Chris Perkins ran, the Paladin's body was replaced by a construct body. If her temple superior would then try and contact her with Sending, by the logic of this answer, the Sending would fail.
A PC wizard may have a clone made and activated when they die. Since the body is not the original body, they would not be reachable by Sending anymore, either°°°.
These downsides are minor - once the other characters interact with their new buddy, it will be familar and reachable by Sending.
It also seems to be a stretch that the spell will enable you to know that the soul changed for someone you just briefly saw. Let's say you see the king in a parade. The king then gets possessed. By this reading, even though you only saw the king but briefly at a distance, and have never talked with him and no idea who he is as a person, since he now has another soul and hence is another creature, you would be able to immediately tell something is not in order by using Sending.
A resulting downside is that this may make it hard to run intrigue and mystery plots. In many such plots someone gets replaced or possessed by the bad guys. As long as the caster knew the original, and avoids meeting their replacement, or suspect there is a replacement and decides to consciously contact with Sending the creature they met back then and are familiar with, the ruse would become obvious. Indeed, if Sending worked like this, what would stop court magicians to cast it first thing every morning and quickly detect if something had happened to the integrity of the king?
° For some constructs it is unclear if they have their own soul or spirit. Animated objects are described as "simple automatons", although those in the feywild seem to be more ensouled. The homunculus "acts as an extention of his creator", modrons "have no sense of self" and "exist as a unified collective".
°° Another question is what happens if you become familiar with the doppelganger, whom you think to be the other creature. By the logic used here, you then are familiar with it, and Sending will go to it instead, unless you explicity try to reach the creature you knew from the past.
°°° In the case of clone, maybe an exception can be made due to the cloned body being a duplicate of the original body.