Enable OpenInClient feature for Onedrives

I found https://support.microsoft.com/en-us/kb/2859986 that details how to enable the OpenInClient feature for a Sharepoint Online site collection and have a few questions. 

Can this be selectively enabled on certain personal sites?

Will this do what I expect, and force Office docs to open in a desktop client?

What will happen if the user does not have Office installed? Will it fall back to using Office Online?

Thanks!