A) When you use DEM as a profile management solution, a local Windows 10 profile would be the easiest, and fastest way (source: https://www.go-euc.com/performance-difference-between-user-profiles/ )

B) Well you could do that, but we implemented FS Logix for the O365 profile. I could highly recommand that, because you could choose to only fit in the Office part of the user profile in a FS Logix container, and manage the other part of the user profile with DEM. FS Logix is included in most O365 licenses FSLogix Overview - FSLogix | Microsoft Docs

Either way, use a profile container for O365 users. If you dont use one, and users are going to search their Outlook for emails, Outlook can get really slow!

C) As i said, i would recommand out of my experience to go with DEM for the main part of the user profiles, and use FS Logix for the Office part of the profile. But Writables would also work ofcourse, but my experience with them arent that great. This because of corrupt Writables etc.

D) Cant say anything about this App Volumes version yet, because we still use the 2.18 version.