No shit Sherlock, that is exactly the point.
Whitebeard
knew that Oden was the heir to the throne.
He also knew that Oden detested Wano’s policy of isolationism.
So it is damn strange that Whitebeard would not find it concerning that his sworn brother in arms and heir to the Shogunate, who hated isolationism and ranted about opening the borders every five minutes, would continue the policy he so openly hated.
It is the duty of someone who claims to be a friend and a brother, ideals that Whitebeard’s character should hold above all others.
Oden did not follow either Roger or Whitebeard as a subordinate. He joined as an equal. Whitebeard outright called him his “brother”, an honour no other WBP was ever bestowed because it shows a spirit of equals. Meanwhile, Roger literally begged Oden to join his crew for his skills, in return Oden got his fill of adventure.
As for Kaido, suggesting that pure blackmail denotes a willingness to comply is an absolute reach. Never mind the fact that once it fell through, Oden nearly outright killed Kaido.
It should be obvious to Whitebeard that Oden would never willingly serve Kaido in any capacity.
These are all irrelevant straw man arguments.
It does not matter whether those villagers mention Oden or not. Fact is, Ace has been to Wano in the last five years. He directly knows how shit the situation is yet apparently the WBPs do not know a thing.
For this to happen would involve a long sequence of unbelievably contrived circumstances in which everyone involved, from the villagers to Ace and the Spade Pirates to Whitebeard to Izo to the rest of the Whitebeard Pirates acting like dementia riddled gargoyles that fail to act like realistic human beings.
Literally all the pieces of the puzzle are there for the Whitebeard Pirates to realise that something is gravely wrong but they have somehow added 2+2 and got -1.
Remember how Whitebeard directly intervened on behalf of his drinking buddy Neptune (who was never part of his crew) regarding Fishman Island?
Remember how Whitebeard was so bothered by his sworn brother leaving to join that “rival” (that Whitebeard was on good terms with anyway) that the first thing he did was to make sure he had supplies so that he would be ok?
“See you soon”, aye Marco?
Again, Whitebeard knows Oden would never serve anyone willingly.
Regardless of whether Oden changes his mind or not, it is still strange that Whitebeard never considered it suspicious since he personally knew Oden, it is still strange that Whitebeard would visit a drinking buddy (Neptune) and a rival (Roger) but not visit his sworn brother in two decades (Whitebeard was literally dying of old age he took that long) despite the fact that Whitebeard prizes family so highly and it is still strange that the WBPs never figured out that anything was wrong despite an avalanche of clues that they should have noticed simply by talking to each other.