is there a way to "un-create" the project so that all the photos go back to the original projects they were pulled from? thanks!
Only directly after creating the project you can use the "undo" command ⌘Z (Edit > Undo ....) from the main menu bar to roll back to the previous state. The most recent commands are kept in the history and you can undo them backwards. If you have not done anything else since since moving your images you can try to undo the last commands, until you reach the command you want to undo.
No, there is no way to undo _Project_ creation other than to immediately undo it. _Albums_ are another story. In your follow-up you ask about an Album. Do you mean a Project?
You will have to manually move the Images into the Projects you want them to be in.
It may be possible to create groups based on metadata that help you figure out which Images to put in which Projects. How are your Projects organized? What metadata have you applied to your Images?