The N64 only adapter fixes the deadzone introduced by VC whereas the N64/GCN combo one doesn't. The singular one is also a bit cheaper.

No one is restricting you from making categories. However, that does not mean there needs to be a leaderboard for every arbitrary category that anyone could possibly think of. There is an additional place off speedrun.com called the various category leaderboard where glitchless along with many other categories are as to not clutter this leaderboard. If something has many runners doing the category, such as any% all keys did last year, it has the possibility to be added as a miscellaneous category (or even eventually as a main category). However, this requires more than one person if interest and there may be reasons for not having the category, such as glitchless that the community knows about that may not be talked about in public as 2dos eluded to.