Defender, as it was called from Win 8 and above, will be shut off as long as another antivirus on the computer is installed. Even though it's integrated, it can still be controlled by the user. It may not be the best free alternative, it's better than nothing.
As far as updating MSE on 7 and older, when I do install it for people, it seems to be slow on the initial definition update, then the regular periodic updates fly by in less than a minute. I would suggest a possible uninstall and then reinstall, that might fix the update issue.
EDIT: According to
this article, if you have no other antivirus running, you can deactivate it on Windows 10, but after a while it will start up again on its own

(WTF?!)
It's bad enough that starting in October they're going to be bundling all the updates for Vista/7/8 just like 10 does now, no control over what to install or skip, that means a lot more likely of a chance for systems to be bricked due to bad updates.