"Codename Brave"
Your argument for this point is that Brave does not match the parameters for someone like Erdrick, and is too generic a term to attribute to any one character, correct? However, let us look at it this way. The character we are specifically talking about when we refer to Erdrick is the protagonist of Dragon Quest III. Said character is of average height and build, equating to about the size of a teenager. With that in mind, I would say it is quite likely for them to have all the stats you have listed. A teenager would certainly be slower than a child (Young Link). Furthermore, the reason we have linked "Brave" to this particular version of Erdrick is because their Japanese name translates to Brave, as you have mentioned. Such a fact cannot be denied.
"Leaking"
Leakers are naturally going to disagree. Not all of them have the same info, and some either have no info at all and simply wish to mislead us, or are being fed lies without even realizing that fact. As such, I agree that we cannot trust most leakers. However, those with clear track records have given their reasons as to why Erdrick will be in Smash. To get into specifics, your assertion that Vergeben getting the Link's Awakening remake wrong means very little in the grand scheme of things. One mistake does not erase the myriad of other things he has gotten right. Ken and Incineroar being the final two characters, Simon and Ridley being in the game, and so forth. While yes, he has not, to my knowledge, named any specific character in his claims of "a Square Enix character is coming", I believe the evidence that points to Erdrick without Vergeben's word is reason enough to assume Erdrick is who he refers to.
"Marketing and Fanservice"
Sakurai had a say in what characters would be added as DLC to Ultimate, yes, but he did not create the list himself. The list of potential characters was bestowed unto him by Nintendo, and he simply chose from who was listed. Furthermore, a character being "unpopular in the West" means nothing. A character being unpopular in any given region means nothing. First off, Smash is developed in Japan, so obviously Sakurai will have his home country's interests in mind first and foremost. Second, characters such as Shulk in Smash 4, Marth and Roy in Melee, and Dark Samus and King K. Rool in Ultimate shows that Sakurai is willing to add characters that will appeal primarily to a single market. Furthermore, I would say Erdrick counts as "unexpected", since, as you have listed, many Westerners do not know of the character. Tell me, would you expect a character you have no knowledge of?
"Complications"
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All I'll say here is that Square being stingy with one property does not mean they will be completely and utterly adverse to adding another. Cloud's lackluster inclusion has no bearing on Erdrick's chances.
"You'll never see it coming"
Using Joker's reveal as a basis for your argument seems foolish. Furthermore, the idea that Nintendo would add data for an entire character simply to throw off dataminers is even more foolish. You speak of the same company that added Roy and Ryu's data to Smash 4 in an update well before they were supposed to be added, and saw no problem. Clearly, they have not learned from that mistake, and those who do not learn from history are doomed to repeat it. This was simply another unfortunate case of them adding a character to the game's data before they were ready to reveal them, only this time they tried to be coy and put them in under a codename, as if that would help matters at all.
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In conclusion, your argument relies on second-hand accounts, personal assumptions, and evidence you cannot back up. While I do not think Erdrick is guaranteed, none of your reasons have swayed me in the slightest. I applaud the effort you put into this essay, however.
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And so, the prosecution rests.