We're aware of the rules issues with rules that span over midnight and this will be fixed.
I think what you ran into is simply just a UI bug and we're still going to improve on that UI, but I'll let the software team know.
And yes, you can edit the Rules.xml file, but please be aware that you can break your rules this way. Worst case, delete the file, create a new rule in the web UI and the Rules.xml file will be re-created. That said, if the backend logic doesn't support your rule, it won't work.