Why not just get Ultimate Windows Tweaker 4 and disable it (among other things)? 
I am actually considering paying to upgrade to Win 10 Professsional. It has the means within it to put Cortana to bed and leave it there.
Presently I do get Cortana to temporarily go away, using task manager to continually kill it until it quits raising its head again. I will get relief for a while, but if my Windows session lasts long enough -like some hours - I will check task manager and Cortana will be live in the background again.
https://www.pcworld.com/article/3109900/you-can-remove-cortana-from-windows-10-but-its-tricky.html
The gpedit function is not available in regular Win 10. I did what was described as work around by downloading the Group Policy program and its folders, from MS, inspite of my system not being in a level higher than Win 10 Home. It did install a gpedit dll, but not the gpedit.msc; and gpedit as a command did not function in any way. It was as if I had downloaded nothing new into the Windows/System and Wondows/System32 folders.
I might pay for the Win 10 Pro upgrade and hope with it I can finally kill Cortana, or at least put to sleep for good.
Why not just get Ultimate Windows Tweaker 4 and disable it (among other things)?