Alternate timeline where Meta Knight is mistaken for the hero of the lower world, and Dedede gets mechanized by Haltmann Co. How would the climaxes of Triple Deluxe and Robobot play out?
That's an interesting role-switch! If I may, I want to ramble how that would affect entire plots of both games, because, I feel like both of these characters aren't TOO interchangeable for both premises to play out exactly the same.
Triple Deluxe
Meta Knight would likely not get captured, because he doesn't seem to have any sort of fear of bugs, therefore Taranza wouldn't be able to catch him off guard like he did with Dedede.
Therefore, Meta Knight and Kirby would team up together to go upwards to Dreamstalk purely out of curiosity of why said attack even happened to begin with. Taranza would realize that he done fucked up and tries to run away from them to warn the queen, but the duo will always manage to catch up and he would have to try to stop them with those classic Triple Deluxe bosses as per usual. When we reach Royal Road we would have to face against Meta Knight which Taranza briefly managed to get under control. Of course, this is futile and Kirby helps him out and breaks him away from Taranza's magic. After that everything plays out similarly to the actual final game, with few notable differences. Cannon section will probably be entirely different, since I see no reason on why Meta wouldn't want to fly out and try to slash those buds with galaxia, considering he literally has wings and he is about as big as Kirby. Meta then frees Kirby from Sectonia's vines thanks to his tornado attack instead of a hammer smack.
I don't think anything else would change, considering both Meta and Dedede are heroes in their own right, so their actions in the climax would parallel each other, with main difference being their approach to heroics.
Planet Robobot
Robobot is interesting, because, provided Susie is completely unchanged otherwise, I think she wouldn't have too much interest in Dedede compared to Meta Knight. She seems to have certain affinity for swordsmen in particular. Plus, Dedede, unlike Meta, isn't as impulsive so he wouldn't go out and try fight off entire army on his own if there are alternative options. So I feel like HWC would capture him as early as the intro cutscene and Kirby's goal would be similar to TDX's where he clearly knows that one of his friends is kidnapped so he goes out to save them as well as free the planet from the Invasion. Let's make it so he wakes up from the noise of the flying cage Dedede put into that leads into HWC Labs.
I'd assume he would become something like Mecha Masked Dedede and due to Susie's affinity for swordsmen she decides that "hammers are too barbaric, compared to the elegance of sword fighting" so she forcibly gives Dedede a laser saber. After we beat him, he falls off the arena shocking Kirby with Susie expressing general disinterest "Oh, I guess even Mother Computer can't teach barbarians of these planet some proper manners. Ciao" and leaves, leaving Kirby and the player for a while to believe that Dedede is done for. In the climax any analogue for Mecha Knight+ fight is absent, we instead fight Haltmann right away.
After Star Dream hijacks Haltmann's body and soul, everything plays out similarly up until instead of the Halberd, what shows up is KABULA! It is Dedede's ship, after all. Revealing that he is not dead at all and he spent all the time in his absence to repair it just in time to help out Kirby! Of course, Dedede does shocked expression as he realizes that instead of jumping into Kabula, Robobot armor absorbs it instead, giving us Kabula mode.
Fight against Star Dream would play out similarly to Halberd mode counterpart, but the way it segways into final sequence is completely different. There is no lever for Dedede to pull. So he does the climax move, gives Kirby a lift with his hammer. With all his strength he smash the Robobot armor with Kirby out of Kabula and on highest speeds he manages to pierce through Star Dream.
Both Dedede and Kirby is later saved by Meta Knight who picked them up with Halberd.