Office 2016 will run alongside Office 2010 and Outlook 2010 and 2016 can use the same data file (but not at the same time). Office 2016 grabbing the default is something you will have to live with for even if you repair Office 2010 making it the default, the next update will change it back again. This does not mean you cannot use the applications of Office 2010. See
http://www.gmayor.com/Toolbars_in_word_2007.htm which will explain how to get the two versions of Word to play nicely together.