How can I dehumanize a selection?

[Liam]

Humanize definitely has undo/redo support. I just tested it myself. Not sure why it didn't work for you.

Humanize affects note times and volumes. Ben's suggestion will help fix the volume. For the note times, you can use this code to snap the notes to the nearest grid line. Set the grid to the finest setting you've used in your sequence, then select all the notes you want to fix. Then open the dev console (ctrl+shift+J), paste this code in there, then hit enter:

tweakNotes(n=>n.time = Math.round(n.time * grid) / grid)

For more info about using the dev console, check out this article: https://onlinesequencer.net/wiki/Console_Commands