A few people I need to share documents with have Office 365, for Word, while my computer has Word 2010. All of us need to open SharePoint 2010 libraries (without saving to our devices) and edit (with track changes), and save directly back to the SharePoint 2010 library. With Office 365 I've gotten as far as opening files in Word but can't find a way to send the edited file back to SharePoint 2010.
Can you make a mapped location on your PC that points to the SharePoint library? This would effectively make SharePoint a place that O365 could see in the computer file structure.
To do this, I open the library in explorer view then drag the icon in the address bar to the favorites section. Alternatively, you could make a mapped drive.