I have the Steam version, which was my original edition.
Someone gifted me the big gift set, which came with another game key so when it went FTP I started a second account. I needed a different email from the main account to do that, plus only the Steam version starts through Steam. The other account I start from a game icon on my desktop which lets me log in to either actually.
I now have 3 accounts, the third works like the second.
Not sure if this answers your question about buying a non-steam version or not but thought I would offer it.
As to the chapter, no idea.. I buy all my chapters through each account page for each account. If I log into the "Steam" account via the account page and purchase any chapters it works fine on the Steam edition. I just prefer to buy that way as opposed to buying through Steam itself.
Hope this helps and doesn't sound too confusing