That plot point about some of Jack's matches being rigged also makes me think if his matches against Himuro and Enjo were also rigged
It's an interesting thing to wonder about for sure. However, I feel like with those two, cases can be made that they weren't forced to lose on purpose based on their personalities and appearances in the show.
For Himuro/Tanner, we know he was the champion before Jack, and we know that losing to this young upstart utterly crushed his spirit. And I personally think that defeat would not have been as harrowing (to the point of driving him into underground duelling and drinking) had it not been real. The part that makes Jack's victory so soul-crushing to Himuro/Tanner is that it was real, I think, and that it was fairly decisive, at that, which he did not expect after apparently having held the champion title for a good while himself.
Then we've got Enjo Mukuro/Hunter Pace, who, if his multiple appearances during the Fortune Cup are anything to go by, seems to have made a habit of challenging Jack multiple times in order to get his title. And you could make a case for that being an elaborate show-gag on Goodwin's part to introduce some sort of reoccurring rival who's always set to lose on purpose, anyway, but you know what kind of speaks against it for me? Enjo's duel against Yusei in the Fortune Cup. This guy is so fired up to try to beat Jack, in a way that I think would be hard to fake.
Plus, there's also something to be said about how the show presents their characters. Through their interactions with Yusei, we see that both of them really value duelling and their pride as duellists. And we get nothing that indicates they might have had a reason to "give up their duellist's pride" by losing against Jack on purpose, like Dragan did because he needed the money for his father's recovery. And I doubt Goodwin could have rigged their duels against Jack without them being in on it somehow, which I simply can't see them agreeing to from a personality standpoint.
That said, I could absolutely see someone taking his idea further in a fic and introducing elements that do force Himuro and/or Enjo to lose their matches on purpose, further undermining Jack's king status and throwing into question whether he ever honestly earned it in the first place. I think it would make for a compelling personal crisis on Jack's end. (Though I'm also perfectly fine with how the show handles that part tbh. Letting him beat Dragan for real and salvaging his pride makes this a good, self-contained mini-plotline imo.)
