Hello Edward,
To answer your questions:
1. Office 2010, they use the following 365 subscriptions (if that clarifies anything):
Exchange Online-subscription 1
Microsoft Office 365-subscription E3
2. Yes, they can save files when they use the Office Web App.
3. Webdav share is a share in Windows to a http location, in this case it is their Sharepoint online location.
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1. Users both meet the minimum system requirements. I can post their exact configuration is you like.
2. I assume you mean uploading to the webinterface? This works fine.
Thanks.