My findings for future reference. Both the .INF update and disabling driver signing were necessary steps:
1. Uninstall broken driver / 'Unknown Device' from device manager if necessary.
2. Update [Manufacturer] section in the .INF file to add a Windows10 option:
;---------------------------------------------------------------;
[Manufacturer]
%MfgName%=Roland,NTamd64.6.2,NTamd64.10
;[ControlFlags]
;ExcludeFromSelect=*
[Roland]
;; Windows7 - Windows XP 64
;; not supported
[Roland.NTamd64.6.2]
;; Windows8
%RDID0015DeviceDesc%=RDID0015Install, USB\VID_0582&PID_0014 ; UM-880
[Roland.NTamd64.10]
;; Windows10
%RDID0015DeviceDesc%=RDID0015Install, USB\VID_0582&PID_0014 ; UM-880
;===============================================================;
3. [Shift] + restart Windows 10. Buried at the end of the resulting page menu is an option to restart disabling driver signing... restart.
4. Install driver as instructed allowing unsigned installation on resulting dialog.