What do you think about a 100% speedrun? I think the problem is that in some levels (at least the snow level) the maximum is over 100% but I'm not sure how much exactly. 100% levels would take much much longer so maybe an individual levels table should be added here for 100% (or whatever the maximum is) completion?

Yeah, I fell like separate tables would be the best for now until the full requirements for 100% run can be established. Then the rules can be set for what substitutes full game completion and then category can safely be made, avoiding all confusion.

Also regarding 100% RTA (uninterrupted full play through), the is the issue of level 3 bonus stage, as it is impossible to complete both bonus stage and 3-2 second part of third stage in single play through. I have these ideas considering that upon successful completion the overall score can carry through multiple games.

1. 100% full completion should consist of two play through of the game first on the beginner difficulty and second on expert, considering bonus stages are the same on both difficulties and it's impossible to 100% complete some of the beginner stages, first play through is beginner any% with 100% completion of bonus stages and THEN it's expert 100% all stages except bonus ones, this way run would cover all the game has to offer, but then again beating the game twice sounds like a lot of unnecessary busywork and no fun to play or attempt for just one extra bonus stage
2. Just 100% expert mode play thought ignoring the level 3 bonus level, because there's no reason to complicate things, and if someone would like to do the first option it can be made it's own category.

So there you have it - #1 offers full showcase of the game but will surely alienate a lot of people and beating game twise for just one bonus stage? Doesn't sound all that fun #2 Just needs to confirm what part of level 3 people are free to ignore and raises the question whether to ignore all bonus stages entirely since might as well as you can't get them all in one going

For level 3 I think you should just go for whichever route nets a higher %. I'm not sure which that is - probably the bonus stage near the boss? Also I wonder if the items dropped after killing a boss in level 3/6/9 are actually counted towards the %? If I remember right they drop in a manner that makes it impossible to collect them all.

However, the rest of the bonus stages should be played to get the maximum % after collecting everything possible in the main level. There's many youtube videos showcasing what appears to be the maximum % so can take those as example.