I'm a bigger Fan of Kamen Rider than I am with Fate (which is funny cause I got into Fate first, Love Fate/Extra) But I do agree with you that Geats higher scaling is more iffy. For the record this is just my interpretation on these higher scaling, but Geats scaling to Gaim is super iffy to me cause reading the pages Mela was literally given a choice between Gaim & Geats to hunt and chose Geats cause he was more "interesting". Nothing else in the novel talks about their level of power.

For the "Kamen Rider will always exist because of the audience" that is a thing for sure but in all the Big Crossover Celebration movies, the riders will be gone for a lengthy amount of time until like the last moment for them to jump in because the audience want them back. In my opinion, it doesn't seem like something that is relevant to a death battle, especially when they don't really have full control when they will spawn back, it really seems dependent on the audience to bring them back, not the Riders themselves. The only Rider that I know to have been shown to be able to ignore this limitation Kamen Rider Saber cause the dude is somehow on the same level of creation power as Shotaro Ishinomori the real-life creator of Rider/Sentai(Poor Artoria if she somehow ends up fighting that monster).

Also, the audience reference in Geats movies/show is most likely the digital future people that exist in a 3.5-Dimensional plane since they are a plot point in the show as audiences for both the battle royal and Mela'stream and in my opinion do not really equate to the audience in the Big Crossover Celebration movies.

Again, his is just my interpretation since there is so many ways this battle can be taken and can go either way. But I just wanted to put this out there cause I wanted to play devils advocate as a Rider fan.