to be more specific, if I played each level repeatedly until I got a good run of that level, repeated that process with every level and then compiled all of the footage of the best level runs together would that be a valid run I could submit?

i guess you can if no one finds out but still dont

No, splicing is commonly attributed as " playing each level repeatedly until you get a good run of that level, repeated that process with every level and then compile all of the footage of the best level runs together " but you make it seem like its a real legit run (as if you did it in one go). So if you say you spliced a speedrun , you pretty much say you cheated. ( or at least i normally use the word like that )

What you want to do is a segmented run. Though ,this also isn't allowed on the leaderboard. We do have a segment category in misc which is each level segment ( sandbox , escape and sky) or you could just run individual levels.