get sh5.exe genericpatcher to work with steam.

[Echolot]

I'd recommend the CD

is there any real DRM difference between Steam, DVD, and Download versions of SH5?

No. If you buy the steam version, you have double DRM, steam and uplay

It is my understanding that uPlay is only required for MP. Is that correct?

You need uplay more for single player campaign to progress but sometimes it seems to not work properly. If you use vpn like tunngle for multiplayer you need no uplay. There are options in TDW Generic Patcher to disable uplay