The “official” names of the labels can be found in the Project → Project Settings → Label List pane. Scrivener does support RegEx searches, but a better solution for repeated searches would be to create a Search Collection.
There’s no automated way to assign labels (or anything else) by outline level, but you can select all of the relevant items in the Binder and then assign the label. If you select a folder document, but not the sub-documents, only the folder itself will get the label.
So, first choose the outline level you want in the Binder, then use the View → Outline → Collapse All to Current Level command to simplify the view.