Simply match up with 2 other dummy accounts. This does involve buying two extra copies on two seperate accounts but if you don't have enough PCs you can always try Sandboxie.

No. Just a good 'ol regular Xbox Series X controller.

Those modes 'sorta' have the reason to be like this since Open is THE PvP mode full of people doing their best to win by both fair and fowl means. Finishing Trial grants lots of points/progression so people of various skill levels are attracted to it, leading to quite random outcomes and the same applies to Horizon Tour but a player-hosted Showcase event? Is this really the place for toxic rivarly? The host (xbqid) was clearly looking for help but here comes that ZIM guy, clearly born in 2023, to be a nuisance on purpose and I couldn't help myself figuring out if they were this bad or toxic on purpose. I suppose the very last act of his/her points toward the latter. Online gaming with randoms... Can't get any more random than that!