How can I dehumanize a selection?

[Ben]

Only manually at this point

Even that doesn't actually work, right? There is no way to manually adjust the volumes of individual notes.

Select all and then do alt 0 to reset all the volumes.

Alt + and alt - can be used to adjust them manually btw.

As for placement, you'll have to just go through and click each note to realign it.
That's odd that you couldn't undo, but for the future make sure you save before and then you can just reload the page.