Thank you, David.
Hi KEvans,
Thanks for your effort and sharing your experience to us.
I would like explain some key points that we need to take care.
1. The URL of the library is permanent since the library was created, it will not be changed even if the name of library be changed.
2. According to the service description of Office 365 Plan P1, the SharePoint Designer is unsupported for P Plan, so we are not suggest using the SharePoint Designer for edit the site in P Plan.
Regards,
Wilfred Ying