Just some thoughts:

If you are going to write anything to SQL Server, write the XLS file itself
rather than some other form it (e.g. CSV, XML etc). An IMAGE (i.e. BLOB)
field can hold 4GB file sizes.

SQL Server, subject to configuration that may be allowed at your site, can
read Excel files directly BUT you would lose the formatting, which you say is
one of your requirements.

Consider writing code in a Workbook to read/write the BLOB from SQL Server
on demand and save it as an AddIn; your users can checkin this Addin in any
workbook and call this functionality. You would need to handle the automatic
archiving of a workbook via Workbook events ... see this topic in the Excel
help files.