Office 365 OneDrive for Business

1) There are retention options, versioning, Recycle Bin etc, there isn't a backup as such, plenty of 3rd party options though

2) https://support.microsoft.com/en-gb/help/3042522/onedrive-for-business-retention-and-deletion

3) At rest and in transit - https://technet.microsoft.com/en-gb/library/dn905447.aspx

4) OneDrive syncing is two-way, save changes locally to your OneDrive folder and they get synced up to the cloud (and the other way round) - https://support.office.com/en-US/article/Set-up-your-computer-to-sync-your-OneDrive-for-Business-files-in-Office-365-23e1f12b-d896-4cb1-a238-f91d19827a16

5) It depends! In some respects, it can certainly replace user drives (home folders) and SharePoint Team Sites/Office 365 Groups for department network drives but there can be challenges.

6)  You can sync SP Team Sites - https://support.office.com/en-gb/article/Enable-users-to-sync-SharePoint-files-with-the-new-OneDrive-sync-client-22e1f635-fb89-49e0-a176-edab26f69614

7) https://support.office.com/en-gb/article/Invalid-characters-in-file-or-folder-names-or-invalid-file-types-in-OneDrive-for-Business-64883a5d-228e-48f5-b3d2-eb39e07630fa

8) https://support.office.com/en-gb/article/Types-of-files-that-cannot-be-added-to-a-list-or-library-30be234d-e551-4c2a-8de8-f8546ffbf5b3#ID0EAADAAA=Online

9) It depends, see the OneDrive for Business limits in the https://technet.microsoft.com/en-GB/library/onedrive-for-business-service-description.aspx?f=255&MSPPError=-2147217396#OneDrive for Business limits

10) Not sure depends on what you want to do, there are built-in https://blogs.office.com/2015/07/16/new-it-management-controls-added-to-onedrive-for-business/https://blogs.office.com/2015/07/16/new-it-management-controls-added-to-onedrive-for-business/and third party tools may go further like Sharegate.  Also, make sure to check out the https://support.office.com/en-gb/article/OneDrive-admin-center-b5665060-530f-40a3-b34a-9e935169b2e0.

11) "Each user in your organization is the owner of their OneDrive for Business site, which is located in the site collection named https://domain-my.sharepoint.com" Admins can grant themselves acces to some one's Ondrive folder as a site collection administrator, this can be done manually or via https://support.office.com/en-US/article/Assign-eDiscovery-permissions-to-OneDrive-for-Business-sites-422858ff-917b-46d4-9e5b-3397f60eee4d

Hopefully that gets you started!