JAAHAS: If I seem unreasonably adamant on skipping the online upgrade before doing a clean Windows 10 installation, it's because I personally have 7 computers at home to update, so the unneeded extra steps would add up to a significant loss of my time even before we are counting on doing the same for my relatives and their neighbor's cat too.
If most of them are stuck on Windows 8 then that's a problem. I still wonder why you need that offline installer. If that's a problem with PC's not being hooked to the internet, then in my experience installing Linux installations need the internet more than Windows ones (though granted I'm sure that can be circumvented).
Anyway, one nice thing about Windows 10 is that PC's can access the update downloads from one another, so you can probably have just one PC set up to update and have the others get the updates over LAN. I'm not totally sure it's that seamless (i.e., that the rest don't have to be connected to the internet at all), but in any case, it would save a lot of bandwidth downloading updates. On the other hand you need to make sure you configure each PC so as not to let the entire world download from you, which is the default.