Completing all challenges, no matter their specific difficulty, deserves in my opinion to figure in the leaderboard if it can be faster than an all challenges all expert run. However, all challenges in expert is in some sense a 100% (achievements are truly worthless), while all challenges but not in all difficulties is a lesser degree of completion.

Among the possibilities :

• Add a "mixed difficulties" possibility to runs, but keep all the "all challenges run" sharing the same subcategory.
• Have a "All challenges (single difficulty)" and "All challenges (mixed difficulties)" subcategories (the names may be improved).

Considering that those runs would be 99% identical, with only the FTL part different (and arguably, the FTL in novice is actually hard...), I'm not really in favor of full separation.

Yes, follow the leader in easy is arguably harder than in expert.