Currently this game is pretty inactive, which is sad, but I've noticed that annoyingly, a large amount of the current arcade challenges are missing.

If these could be added, that'd be great! Score challenges can be added as highest score if possible. It'd also be really nice to have the new challenges currently in the Duck Game Alpha, but if they can't be added (due to being in the alpha) then that's fine.

Score challenges have always been a difficult case; speedrun.com as a site isn't really meant for (nor does it really handle well) runs/challenges which are more high score focused than speed focused. Fortunately, there's been chatter about a potential for Landon to add time-challenge modes to existing score challenge levels; i.e. "how fast can you earn the Dev medal on Mace Face?" If this ends up implemented, it'd be easy and sweet to set up categories for the score levels, but as it stands, it's not really in the spirit of the leaderboard.

To your second point, the new challenges are planned to be added as soon as they're fully released.