Annoyingly, newRecord.getFields() only gets the fields that have a value in beforeLoad. Looping them to set all fields to INLINE will naturally not include fields that doesn't have a value. Instead, I ran a query to get all the custom fields from the table CustomField. Is there a better way to get all fields in the form in beforeLoad?

Are you trying to prevent any edits in edit mode? Maybe you could just redirect to the record in view mode instead?

Yes, I'm trying to prevent edits, but I have a custom sublist with checkbox:es that needs to be available for editing. So I can't lock down the whole record completely. The user needs to validate a bunch of lines by checking those boxes and then submit the record. It was fast enough with the query-method in this case.